To elaborate a bit on Kenm’s answer, the oil in the engine collects at the bottom (it’s more of a pan than a tank). From there it gets pumped up through the engine, where it lubricates all of the stuff that needs lubricated, and eventually drips back down to the bottom of the engine, where it starts the cycle again and just repeats over and over.
The oil in the pan sits below a bunch of moving parts (crankshaft, pistons, etc). If the oil level is too high, then it comes into contact with these parts and the oil gets whipped into a froth. This frothy oil won’t get pumped properly (it’s more air than oil in the froth) and so ironically the engine gets starved for oil even though it has too much oil.
Since the end result is the same (no oil gets to where it needs to go) it’s basically just as bad for the engine as running it too low on oil.
I’m not sure what you mean by efficiency. If you’re talking about flow rate, that is the problem. Thicker oil may not get delivered quickly enough to lubricate enough. This is particularly a problem with cold engine starts, where the “W” factor (the W stands for winter, a cold spec as mentioned by bump) is important.
It used to be that higher numbers meant more protection, especially at high temperatures and high engine speeds. As oil has been improved over the years, protection has been increased by refinery-installed additives such as friction modifiers and film strength enhancers. Today’s 20 may work as well as, say, the 40 of yesteryear. The trend has been to lower numbers. The once common 10W-30 gave way to 5W-30 for many engines, and in recent years we’ve seen 5W-20 and now even 0W-20. Better oil flow, especially when cold, has been found to yield a significant improvement in engine longevity.
So what is the downside of being more viscous? What harm gets done? Potentially a little less protection in cold weather starts? Anything else?
My efficiency unerstanding comes from the fact that my mpg readings on moment by moment reads (hybrid cars for 11 years) are alway lower for the first portion of the drives and then get much better. I was understanding (perhaps mistakenly) that that was because the oil was more viscous at first and therefore everything a bit harder to push around and that as the engine armed things were slipperier and thus more efficiently running.
Potentially, an oil that is too viscous won’t pump properly through the engine leading to more wear and tear on parts and more heat being generated due to friction, possibly causing parts to fail sooner.
And yes, there’s a slight efficiency hit from the extra effort required to slosh the thick goopey oil around through the engine. It’s more noticeable on smaller engines (like motorcycle engines). However. most of the efficiency loss in a cold engine comes from the cold combustion chamber leading to incomplete combustion rather than efficiencies lost due to oil viscosity.
You may already be aware of this, but note that the W number is the “winter” number or the cold viscosity number, so 10w30 and 10w40 both provide exactly the same viscosity (and therefore protection) during an engine cold start. It’s only when the oil heats up that the 10w40 is thicker.
First, engines of today are built with much closer tolerances, engineered better and don’t wear like engines of a few decades ago. When an engine wears it starts to burn oil. The thinner the oil, the faster it will burn off because it will be easier to bypass the piston rings due to how thin it is.
STP was/is highly viscous. It was designed to increase the viscosity of the oil and therefore provide better lubrication and lower oil consumption for engines that were experiencing wear. For the racing engines of the time it also seemed to provide better lubrication under severe conditions.
So when your mechanic said to go to 20/50 it was much the same as the mechanic of the past saying, “add a can of STP.”
If you live in and area with extremely cold temperatures you might have problems with sufficient lubrication upon starting the engine. Otherwise, you are getting a lot of miles out of the beater without having to do an engine rebuild by using the higher viscosity oil.
I do not know where this “common wisdom” is coming from, certainly not the car manufacturer nor a reputable engine shop. It is not wisdom, nor is it common, at least among mechanics it is not.
As has been said, It sure is not good to run an engine two quarts low. In modern engines the oil capacity is less then it was two decades ago. These engines also run at higher RPMs then they used to, add a turbo and you can see that these engines run at a much higher temperature then the the earlier ones did. These engines need all of the cooling that they can get! One should NOT make a practice of running an engine low on oil. Unless of course, one wants to experience a catastrophic engine failure.
Many have replied to you about the frothing issue. I concur with their analysis. Another issue If one were to over fill the oil sump, is that the crankcase pressure may build up to a high enough value that some of the oil seals in the engine may fail. A leaking front or rear crankshaft seal can sling out three quarts of oil in less then two minutes. BAD, very BAD.
I have seen this, when my sister had a car that used two quarts of oil between oil changes. She got tired of checking the oil at every fill-up, so she just added the two quarts to the crankcase just after the oil change. Her car now had seven quarts of oil instead of five. She thought that she was good to go until the next oil change. In practice, the engine oil was high enough that it was churned up by the crankshaft. The front & rear oil seals failed and dumped three quarts onto the exhaust pipe. The billowing cloud of blue smoke was enough of a clue to her, that she shut off the engine and coasted to a stop. Then she called me. I got my trailer and “rescued” her, she was two miles from where she had added the extra two quarts. I got to rebuild her engine sooner then either of us would have liked.
Please keep your oil level where it belongs. No more then one pint low should be a hard and fast rule for you. One pint over full should be OK, no promises on this though.
Depending on what type of driving you do, you could have a serious leak while also not leaving significant drips of oil on the ground. The leaked oil could be coating the outside of the oil pan or other underside parts of the car. It could be blown back and dried underneath your car.
If you can crawl underneath with a flashlight and look, that would be valuable to see if you do have visible leaks or not. A simple and quick way, if you don’t have ramps or jack stands, is to park at a clean curb, with one side of the car up on the curb and the other side’s wheels on the street. That is usually more than enough room to get underneath for a look. Whatever you do, do not crawl underneath if your car is up on its jack – very risky.
I could see this being a problem if the oil level is so low that the oil pump starts drawing in air. But if the pump’s pickup is perpetually submerged, then there shouldn’t be a problem. Having said that…
There’s the rub. If you’re below the “low” mark on the dipstick, you don’t know how low you can go before the pump starts sucking air and causing cooling/lubrication shortfalls. It’s a bit like going faster than the speed rating on your tires. Yeah, there’s some safety margin, but you don’t know how much, so you are basically rolling the dice. This is especially true once the level falls off of the bottom end of the dipstick, at which point you have absolutely no idea how much oil you actually have.
I can’t think of a good reason to operate the engine with oil below the “low” mark. The cheapest API-certified motor oil at Wal-Mart is a couple of bucks a quart, and no driveable car burns/leaks oil so fast that you can’t make it to the nearest store when you notice you’re getting close to the low mark.
I once had a beater, “fill it up with oil and check the gas”. Never a drop on the ground and not that much burning. I “Gunked” the engine and found the oil blowing out the oil pressure sending unit near the top of the engine. Seemed it only leaked when I was driving. Off-topic, but my 4-year-old was trying to be helpful and filled up my gas tank for me … with the garden hose … ROFL … it was sooooooo cute.
After thousands of hours of recreational reading on motor oil as a topic…
They’re generally a waste of money if your car doesn’t call for them and you don’t race it.
They’re also a break-even proposition if you run them and do extended oil change intervals, complete with getting your oil analyzed once you’re done with it, etc.