given the age of the car he might not have the manual. I think we did kind of go too far out to left field before answering the question asked.
I see what you did there. I’ll be keeping an eye on you. 
Also, as OldOlds demonstrated, people seem to believe that the advice in the manual is optimized for when the car is brand new. So therefore there might be a legit reason to use a different oil once the car is older. Info the manual would likely omit since it’s not helpful from a marketing POV.
Given the long-standing success of snake oil advertising in the car lube business (e.g. http://www.marvelmysteryoil.com/) and the modern moves by the existing mainstream oil brands to introduce “high mileage” versions of their oils, it’s hardly surprising that folks might ask for info beyond that found in the manual.
IANA lube engineer. Far from it. But as a long-ago motorhead who still drives performance cars and reads heavily but doesn’t wrench much any more my personal POV is:A) The only GQ answer is “whatever the engine manufacturer says.”
B) Any other supposedly factual info is so tainted by marketing hype and Old Wive’s Tales as to be no better than rolling dice.
C) Excluding Artic or Middle Eastern extremes of temperature, any difference in oil performance in any given vehicle will be indistinguishable. The engine either keeps running or fails. You’ll never have a control for your experiment so you’ll never know whether your choice of oil helped, hurt, or had no effect.
D) It’s far more likely that oil and especially filter changes at the correct interval* are more beneficial to old engines than are any particular viscosity.
IMO. YMMV.
=======
- Danger Will Robinson! Danger! Religious flamewar ahead!! Danger! Danger!
Over my life time with ICE’s, I have found that more often than recommended oil & filter changes did more for engines than brand or additive changes ever did.
YMMV
No, it will behave like a 30, not like a 30W. That “W” stands for “winter,” not for “weight.”
Those periods are probably the most significant – cold engine starts and warmup.
I think OldOlds has demonstrated he’s mostly worried about the oil having too little viscosity, not too much. Which is mostly backwards.
Crudely stated: thin oil can still lubricate; thick oil can’t even get to where it’s needed.
It’d be nice to get him to state what he thinks his choices are and what are the good and bad points of each as he sees them. Then we can tackle any misconceptions. As it is we’re guessing about what he’s guessing about. Which is not a recipe for anyone giving clear advice clearly stated.
Ditto for OP dougie_monty.
Well, ignorance fought. As a guy who’s pretty accomplished with wrenching, I’m embarrassed to say i didn’t know that. I thought it was that the oil would behave as a 5 weight when cold (i.e., startup) and a 30 weight when hot (operation).
It is conventional wisdom that with older cars things loosen up and oil consumption increases. It is also conventional wisdom that a thicker oil will better resist the increased consumption that can come with this. So many people run 10W40 and 15W40 in their old beasts for this reason. It seems to help, but as a scientist I would be remiss if I didn’t acknowledge that i’ve never done any experiments to verify (which would be far more effort than I am willing to put into it).
Mine is a 110,000 mile car that I believe has never had the engine opened up- the rings and seals are as from the factory. And it does like to eat its share of oil, mostly from valve seals and a steady flow from the rear main seal. There’s also evidence that someone hit something with the oil pan because there’s a little crush next to the drain plug which keeps that from sealing completely.
(boy, that all makes it sound bad, but she really runs quite well)
A rebuild is on the list for when I get a heated place to do it in the winter, OR when it finally gets so bad that I’m not enjoying it in the summer.
I’m putting in about 1qt every 500 miles which is manageable for a play car but obviously higher than one would like.
There’s a lot of good approachable explanation of grades and thicknesses at Motor oil - Wikipedia. Plus the usual industry insider mumbo-jumbo alphabet soup that can be ignored.
I’ve had cars just like that. Including the dented pan that makes it sorta scary to pull the drain plug for fear it’ll never go back in. Having that plug be the lowest-hanging part of the car was not a good design decision. 
If you’re trying to keep the beast running and you live in a place where it gets cold, I’d be real leery of running 15W-anything. At this stage of the car’s life it needs all the help it can get to have enough oil in the critical first 30 seconds of operation when all the wear happens. Doubly so if you don’t drive it much so it sits for days between starts. Between sludged-up oil galleys and a tired pump that bypasses a bit, your actual delivered oil volume and pressure at the bearings isn’t what it was when new.
IMO 5W-30 will keep your engine alive longer than will 10W-40. At the expense of *maybe *burning more of it. You might be surprised at how small the difference is. Truth be told, oil that leaks out isn’t bad for the engine; it’s just a slow stealth oil change. What’s bad for the engine is oil that’s burned. Both are environmentally unfriendly, but we can ignore that for discussion’s sake. Until you know how much is going out which way you don’t really know if thicker oil will preserve your rings & valve guides or not.
My last car had 185K miles on the original engine and a bad rear main seal. It went through a quart every 1200 miles or so. As best I could tell it didn’t burn a drop. It made more sense to me to keep running the factory recommended 5W-20 than risk having crappy lubrication at start.
Again this is all shade-tree anecdote and worth every penny you’ve paid for it. Good luck.
I hope your '67 is a Toronado; those were really cool cars. They also made some neat coupes. Their sedans: not so cool.
You have the concept right, the mix-up is in the terminology. Most people have heard oil referred to as “xx weight,” and it’s logical to assume the W stands for “weight.” Logical, but wrong.
The whole point of multigrade oil is exactly as you say, essentially meaning flows well when cold and also protects well when hot. It might be clearer if we exaggerate the name like this: 5W/+/30, and state it as “rates a 5 at Winter temperatures, but rates a 30 when warmed up.” It’s a little non-intuitive because the cold rating is indicated by the W, and the hot rating is indicated by…nothing. If they called it 5W30H (W=winter, H=hot), there would probably be less confusion.
Agree.
When it comes to blended/multigrade engine oil, some people misinterpret the numbers and believe the oil has less viscosity (is “thinner”) at cold temperatures and more viscosity (is “thicker”) at high temperatures. Take 5W-40, for example. Some people will say, “It has a viscosity of 5 at cold temperatures and a viscosity of 40 at high temperatures, which means it gets thicker as the temperature increases.” This is not correct. All engine oil, regardless if it’s “straight” or blended, is thicker at cold temperatures and thinner at high temperatures. You wish it were the opposite.
This chart explains it pretty well. As can be seen, straight SAE 5W oil has a low viscosity at low temperatures (good), but its viscosity is too low at high temperatures (bad). Straight SAE 40 oil has a high viscosity at high temperatures (good), but its viscosity is too high at low temperatures (bad). By blending them, you get the best of both worlds. At low temperatures, 5W-40 has the same, low viscosity as srtaight SAE 5W oil at low temperatures (good). And at high temperatures, 5W-40 has the same, high viscosity as straight SAE 40 oil at high temperatures (good).
I have consulted my owner’s manual and I am baffled: No specific grade of oil is mentioned. I suppose I will have to contact the service department of the local Mercedes dealership for this information…
So in an older vehicle, or in my case, boat. The manual calls for SAE 30. This is for a 1979 mercruiser, uses an old chevette engine I believe. Would it be recommended that I use a 5w30 instead of the SAE 30. Keeping in mind, it’s for a boat, so it’s only being used in warm weather, which is why I assume SAE 30 would be ok.
I do love the gen 1 Toros. I had the idea once that someday I would buy a complete set of 67 Oldsmobiles… But now that I’m middle aged and could actually do that I seem to feel it’s more trouble than it’s worth.
Mine is a 98 Convertible. I have always had a soft spot for the convertible barges of the old days. It’s got the high compression 425 4-bbl, which at this point puts out about 120 HP, I suspect
In late HS I “inherited” my Dad’s 69 Cutlass Supreme. Sadly, it wasn’t a convertible. But it was *tres *cool for the time. He upgraded to an Eldorado which, despite the fancier make, wasn’t quite as cool as the Toronado of the same era.
If you’re gonna drive a full-up barge, it’s gotta be a cloth-top. You’ve certainly got a classic there.
5W-30 signifies the temp range. 5W means it’s 5 weight in Winter temps while 30 weight is at operating temps (engine heat). You may say cold start but it’s not always 5W at cold start as it depends on the temps. On a Summer morning it can be at 10-15 weight for example.
Except maybe castor oil, i have never seen a motor oil that would be the consistency of a fruit rollup even at 70 below.
If you drained it hot into the snow, it would melt the snow and you’d have an environmental mess of oil and water.
That reminds me of a tall tale my Dad once told me:
He was in a mountainous area in a cold winter, and set a bucket of boiling water outside. It froze so quick the ice was still warm.
Just as a data point, my 2004 Ford Fi50 with the Triton 5.4 liter V8 calls for 5W-20 oil designed for gasoline engines. It does not specify synthetic but the Ford part number is for synthetic blend.
I wouldn’t put it past a German car company to specify their material specification instead of a specific grade, e.g. “Use only Motorschmeierstoff 5.22.A3-11C in your Mercedes-Benz vehicle.”