Oil Viscosity For Old Cars

The reason you want a low viscosity multigrade oil is to provide adequate lubrication/oil flow when cold after intital startup. There is nothing to be gained by using anything heavier than 10-40 in a good engine that does not suffer from excessive oil consumption. As a matter of fact fuel economy will suffer slightly by running too heavy an oil. I live in a warm climate (SanDiego) and I use 15W-40 in both my MB's.( 190E 146K miles, SL320 105K miles) We recently purchased a new Hyundai SantaFE with the 3.3 Liter engine and it calls for 5W-20 oil due to the tight aluminum block clearances. Hope this helps

Peter

However every lube shop I go, they recommend using 20w-50 viscosity because of the age / mileage of the car. They say that due to engine wear, a car with more than 60 kmiles should use a thicker oil.

It makes sense. It is just that this car does not show any sign of wear yet, at least the ones that I am aware of, blue smoke exhaust and increased oil consumption.

In that case, and according to what I know (though not much and just theory), the thicker cold viscosity (two times thicker) would make the cold oil harder to flow during winter cold starts, increasing the wear. Also the thicker hot viscosity would cause the engine to generate more heat (and hence be less efficient).

What do you think?

Thanks Marcelo

If the car does not use an excessive amount of oil, then I would run the lightest recommended oil. In this case the 10w-40. Heavier oils are generally used to slow down oil consumption.

EJ > The reason you want a low viscosity multigrade oil is to provide adequate

I agree with everyone here. Go as light as you can... like 5W40 or 0W40... synthetic or not... it is your choice. I find synthetic oil are much better on older engine if you are not leaking oil.

If you were burning oil a heavier grade would be a good idea, but most Benzes are running well long after lesser engines have gone to the yard. If you are not burning oil, stay within published ratings, but be warned that the newest rating, "SM" has far less zinc to protect catalytic converters. If you have a diesel, or any pre-cat four-stroke engine, use Delo, Rotella, Motorhead or old stock of "SL" or earlier oils. I work as a machinist building parts for vintage Lotus racers, and many vintage racers are reporting camshaft damage with the new oils. Many Benzes were susceptible to camshaft/chain wear with the good oil. Modern engines designed for thinner oils (5W20, etc)will probably not survive with 20-50, even if they are burning oil. It will cavitate in the bearings.

Who do you think knows more? The company that built the engine and makes the oil recommendations or the local high school dropout grease shop monkey?

I discussed this with an engine builder a few years ago who is very respected in Nascar and SAE. He's lectured at MIT and other places. I can't remember his last name, but his first is Jan. Anyways, he said to run what they call for since the engineers have determined that weight based on clearances at all operating temperatures, and if you stray in one direction it is better in his opinion to stray in the lighter direction if you want to go faster and save gas, but if you go too light(a few grades lighter) you will be risking excess wear. He also had the opinion that the only acceptable time to use thicker oil is if you are burning oil and are trying to keep the oil from burning. He equated that to a duct tape fix though since it is just going to make the matter worse because the heavier oil will cause excess wear.

Apparently among mechanics it is thought to be the thing to do to run heavier oils in old cars. This is only because in reality many of them that the mechanics have probably owned need it because they are in need of a rebuild. So the mechanic with an old clunker, or seven, gets used to old cars needing heavy oil, then it becomes lore that all of them need it regardless of whether or not they are actually burning oil.

My first car was a 71 pontiac with over 250k on it, and it would burn through a quart of 20W50 every 500-700 miles. Sold it for 100 bucks to a guy who restored it and then had a very sweet car. Oh well.

You can't go wrong with synthetic oil. The property is far superior to dino oil.

Majority of engine wear is during the engine startup... so which is better? Thick dino 10W oil or thin synthetic 0W oil in cold weather?

It is not rocket science.

Thanks for the answers. It is clear that 20w-50 is not the best for this car.

I=B4m planning to keep this (inherited) car for a long time, and I would like avoid its engine wear as much as I can.

So I have one more question: If the viscosity prescribed by who built the engine is according to its tolerances or clearances between parts (cylinder - piston rings), then could the use of a thinner oil (0w instead of 10w), though good for cold starts and less wear, result in more oil combustion and consequently more oil consumption? If so, is it important?

Thanks for your help.

Here's a few links regarding lubricants, but bear in mind that they are a few years old and the Zinc issue is a new one.

That ought to kill a few hours for you.

Yes, it could.

Only if it makes a significant difference. My 1980 300SD 120K miles has 10-40 in it during winter, 15-40 during other times and it has never needed a quart in between changes.

Usually the manufacturer recommends a range of viscosities for the same temp range of operation. . If the car is starting to use oil, then I'd go with the heavier one within that range, especially for warmer weather.

You also have to remember that the typical grease monkey thinks a car with 100K miles on the engine is over the hill. With a MB, it's just getting broken in, which is a good thing :)

You MIGHT get away with it. This question has been bedated more or less non-stop since the invention of the internet and answer remains the same: use the oil the manufactur recommends.