I know there are varying weights, but lets say i use 5w30.
What’s the difference between the stuff i can buy for $1.49/qt and one for $7.99?
$6.50 more out of your pocket.
IMO, there aren’t any significant differences between the oil (except the brand name, and the price tag). Each brand may have different additives for cleaners and de-icers, but those all seem to be pretty similar. in function.
Synthetic oil vs. natural petroleum oil is said to make a difference for some uses. Personally, I’ve never noticed any difference in use.
Honestly, not much of a difference. As long as the oil is clean. Synthetic can have a positive performance impact on some engines. IMO its more noticeable on cars with 4 cylinders and 6 cylinders, most good quality cars with 8 pretty much require synthetic only anyhow. Even then, the impact is negligible. The only thing i’ve heard is switching between different oils (standard vs. synthetic) can negatively impact gaskets and seals…that has yet to be seen. Personally, I use a synthetic blend and i get an approx 1.3 mpg benefit from using it over regular, but have to use 10W30 (older car, worn = thicker oil), even though my car specifies 5W30 so i am just compensating (heavier weight reduces fuel economy is some cases). Use whatever is cheapest that is specified for your car, probably a good idea to avoid dollar store oil though. No evidence in this case, but not willing to chance it.
Even some “neutral” sites such as Angie’s List seem to like synthetics
https://www.angieslist.com/articles/synthetic-motor-oil-better-your-car.htm
I like them for higher mileage vehicles and/or high performance. Basically in its simplest form synthetics are distilled natural mineral oil. Yeah, its more complicated than that but that is the basics. The more refined oil can be advantageous depending on your motor and how you use/treat your car.
For cars under warranty, do whatever is necessary for the warranty. Follow the owner’s manual/oil life indicator.
Oils, engines, and how engines are used are all different. As long as you change the oil regularly you’ll be fine.
How long “regular” is depends on the oil, engine, and how it is used. On one of my cars, I can do a synthetic oil change once a year or conventional every three or four months. Synthetic costs me about 2-3x as much so it is a better economic choice for this particular car. There may be some brand of conventional that would last a year, and maybe my unscientific “look at how dirty the oil is” isn’t the best way to know when it needs to be changed, but Pennzoil Platinum or Mobil 1 are about $5/quart at Walmart…
Conventional oil should be changed every 5000 miles or more, check your car’s manual. 3000 miles has not been true for at least 2 decades, but is a common myth still. It’s good to do if you really hate money. Synthetic can be changed less often.
My car is from 1999 and recommends 5000.
If your car doesn’t demand synthetic (your owner’s manual will say if it does) then you really don’t need to spend more for it. I had some quick-lube schmuck try to up-sell me on $12/qt synthetic for my SRT-4 because “you know, these turbo engines” which I dismissed with “it’s gone 165,000 miles on conventional oil, it’ll be fine.”
however, some cars now are calling for oils like 0W-20 or 5W-50 which can only be made out of a synthetic base stock.
honestly, what motor oil does is something you aren’t going to “personally notice any difference.”
Just do what the owner’s manual recommends.
Blackstone Labs looked for residual metal, which indicates engine wear, in used oil samples from Subaru engines. They found essentially no difference in wear between any of the oils.
https://thegarage.jalopnik.com/why-expensive-oil-is-a-waste-of-money-1797241527
I should take this advice. I use synthetic and change it every 5000 miles even though my car only calls for conventional oil and change intervals of about 9000 miles based on the oil life monitor.
The requirement for specific oil type and change interval varies from car to car. E.g, the factory fill oil on the C7 Corvette is Mobile 1, and Chevrolet recommends the initial change at 500 miles: https://thegarage.jalopnik.com/chevrolet-tells-corvette-z06-owners-to-change-oil-after-1724296112
Likewise it’s the factory fill oil on Porsches: https://mobiloil.com/en/article/why-the-mobil-advantage/automakers-trust-mobil-1/porsche-recommended-oil
This doesn’t mean Mobile 1 is the best or should be used in other cars. Factory fill agreements can be due to business partnerships and co-branding, not just oil performance. However Chevrolet and Porsche obviously trust that oil in their expensive engines, on which they (not the oil company) must provide warranty support.
If I had a regular non-performance car and drove in normal conditions, I’d use whatever oil met the spec in the owner’s manual, and use the recommended change interval. However that recommended change interval is typically 2x as frequent if the car is driven in cold conditions, dusty environment, for short trips or in stop-and-go traffic.
The only way to be sure is have oil analysis and professional interpretation done. However this is more expensive than just changing the oil at 2x the recommended interval.
If I had a performance or modified vehicle, I’d have oil analysis and professional interpretation done by Blackstone https://www.blackstone-labs.com/ or (preferably) Dyson: http://www.dysonanalysis.com/
Some of it depends on the vehicle. Some engines will sludge up more easily with conventional oil vs. synthetic. With other engines, it doesn’t seem to make any difference at all. With many engines, as long as you have enough oil of roughly the right weight (doesn’t even have to be what the manufacturer recommends) the engine will get enough lubrication and everything will be fine.
I did kind of an experiment once. I had an old Nissan pickup truck that I really wanted to die so I could buy a newer truck. The body was rusting off of it (a friend of mine nicknamed it the Bondo Bandit), the windshield leaked, and it had a bunch of little annoyances on it that weren’t worth fixing. So when it hit 200,000 miles, I stopped doing all maintenance on it and waited for it to die. I did not change the oil at all. Five years later, at 260,000 miles, the engine was still running like a champ. The fuel pump finally died on it (nothing to do with the lack of maintenance, it was just an old part that wore out), and while I could have easily changed out the fuel pump, I used that as an excuse to junk it and finally got a newer truck. I had always put the cheapest oil I could find in it, and it went for 5 years and 60,000 miles on the cheap crap with no oil change.
Note that I don’t recommend that you do the same sort of thing with your own vehicles, unless you want them to die too. 
On the other hand, we had a Toyota Camry that had been passed down from my mother to me to my wife and finally to my son. He went a bit long between oil changes, and the el-cheapo oil in that car sludged up and clogged the oil filter. Synthetic oil in that car may have avoided the sludging under those circumstances. Changing the oil more frequently would have avoided it as well.
I recall decades ago either reading or watching something on a study done in New York City taxis. The end result was that as long as you used an API approved oil rated for passenger cars (S rated), the correct viscosity, and changed it as per the recommended frequency, there was no difference in the performance of cheap to expensive oils.
I can’t recall if synthetics were part of the study, but I don’t believe they were. I don’t believe synthetic oil does a better job as long as the above criteria are met.
I have been told by dealer mechanics (Ford and GM) that while the kind of oil isn’t critical as long as it is within spec and has the proper rating, manufacturers have noticed that switching between oil brands (i.e., their additives) will increase the sludge. So, pick one and stick with it.
In my reading on the subject, I found that there are more significant differences in the oil filters. To my surprise and disappointment by my long-time choice of FRAM apparently is more advertising than technology. The best filter I have found (for some amateur value of best), is WIC.
Also, one significant difference in oil is the API oil grade. The standards change over time, later letters are newer. (I believe the latest oil grade for passenger cars is SN-S for light duty service and N is the incremented letter) One way the discount oils keep their prices low is to sell older standard oil. As far as I know, later standards are always better for all cars. For instance, the latest standard explicitly addresses issue caused by E85 gasoline.
As noted above, there is no difference as long as it meets the API spec for your engine. However, I run synthetic in my car because of the winters in Alberta and synthetics have a better pour point at very cold temperatures. If you have a vehicle that runs under extreme conditions of heat, cold, or things like off-road or racing regularly you will probably benefit. It’s pretty unlikely that you are going to see a big difference in your day to day commute. If you really want to geek out about this subject I recommend you visit Bob is the Oil Guy. Be warned that there are audiophile levels of opinion and snark in the forums, though.
I’m curious about this. I can understand the type of oil causing more or less wear on the engine or more or less sludge deposits, but I don’t understand the mechanism whereby the type of oil can affect gas mileage.
lower viscosity oil takes less power from the engine to pump it but I can’t imagine going from conventional to synthetic would gain you over 1 mpg.
It should be pointed out that much of the difference between conventional and synthetic oils is not how well they work when new, but how well they work when old. Synthetic oil is more resistant to thermal breakdown, and will maintain its desired viscosity and other properties for longer and in worse conditions than conventional oils.
As others have said though, if you’re using the oil weight recommended by the manufacturer, and changing it at the manufacturer’s recommended intervals, then it probably doesn’t matter. My vehicles have all recommended synthetic, and it’s not worth the savings of $20 for the once per year oil change to try and get away with something other than what’s recommended.
which is why I use synth. in my air-cooled motorcycle; those engines get hot.
I actually make the additives and additive packages that go into motor oils. There are differences in motor oils; it’s debatable how much you should care about those differences.
First of all, the synthetic question: echoreply has it correct, in that the main difference of concern is that synthetic oils are much more stable to degradation. The data on sludge formation is pretty convincing, so I use a synthetic oil myself.
Next, oil weight: the engine manufacturer designs the engine tolerances with a very narrow range of viscosity for optimum performance. The recommended oil weight is the result of them trying to keep you in that optimum range. It has very little to do with the performance additives in the motor oil (other than some solubility stuff). You should use the recommended motor oil weight for your engine for best performance.
The additives in each motor oil will meet a set of industry or company standards, which will be cited on the label. Two motor oils meeting all the same standards are equivalent, and you should not purchase the higher priced of the two. There are different standards for the US and Europe, for example, although many motor oils will be spec’d as meeting a group of these. These standards advance over time as rbroome references for API; ILSAC is currently on GF-5, and we’re working on GF-6. If you’ve got an older car, you can get by with an older (probably, cheaper) grade. Certain markets – Latin America, Middle East, Africa – generally will be using older grades while the US/Europe market has upgraded.
Also, some motor oil manufacturers try to differentiate their product by imposing additional performance requirements, and so will have developed their own standards. Likewise, some car manufacturers try to claim higher performance with the requirement of using a motor oil meeting their own, additional performance requirement standards. If you look on a label and see a company-specific standard, that is where this comes from and the additives in that motor oil will differ from a motor oil that only meets the market general standards.
It will depend on the customer’s specific requirements as to how each differently spec’d motor oil will be formulated. It may have more dispersant, or a different viscosity index improver, or more antioxidant, or whatever. The “max life” motor oils tend to have more dispersant and detergent in their formulation, to deal with higher amounts of sludge & soot seen in older cars. These motor oils would reasonably cost more than a market general motor oil. If you want that additional performance, you should expect to pay a higher cost.
It is debatable, though, whether the more stringent specifications and higher costs are worth it. Most drivers do not need a high-performance motor oil, and should be fine with a market general offering. Some of the company-specific standards are intended mostly to differentiate their product, and may not be providing an actual benefit to performance. I’d be skeptical of any claim that does not include an actual performance benefit – if they’re advertising what is in the oil and not what it does, Marketing is involved. And some of the engine performance tests are notoriously dodgy in the industry, so even if they are advertising increased performance, judge it with a critical eye if it’s costing you more for that.
In short, unless you’re driving an extreme, high-performance car, go with a market general offering of the manufacturer’s recommended weight. Look for the API or ILSAC grade, and use the one in your owner’s manual. Maybe consider a synthetic. Or a “max life” if you’ve got an older car.
By the way, the oil change interval is based on how long it is estimated that the detergent in the oil will still have some basicity. It’s mostly the overbased detergent in the oil that neutralizes the acidic byproducts of combustion (&tc.); once your basicity is gone, time to change the oil. Most of the other additives are present in enough quantity to last beyond that point. There was less detergent in formulations when 3000 miles was the recommended change interval; current formulations have more base. I usually change every ~6000 miles. You can buy top-treats that are mostly detergent (e.g., used in semis), but that’s probably a bit more complicated than most car users will want to deal with.
Ref the first snip, this is why we love the Dope. “I do this for a living and …” is such a powerful endorsement for your info. Thank you.
Ref the second snip, I really like the idea that the litmus test for when to change your oil is … well … a Litmus test. 
Who knew? Not me, despite being a gearhead for many now-bygone years.
Probably the best part of my job is going into a mechanic or car dealer who tries to upsell me on whatever fluid service they’re trying to peddle, and getting to tell them “You know I make this stuff for you all, right?”
A few years back, GM started sampling all your fluids when your car was in for service to scare the customer with all the dark sample vials they’d pulled. It was hilarious when the service guy mixed up the transmission fluid with, I think, the wiper fluid. And tried to sell me on a transmission fluid change because it was “off color”.