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Discussion Starter · #1 ·
Here's a post I made on the old VMF a while ago. I want this post to continue to be of use to anyone visiting the NewVMF and when the old VMF has finally closed.
I used the search feature to find some articles related to engine oils and found so many different answers, opinions and contradictions that I decided to post something about this issue.

The all-familiar SAE viscosity ratings are not as simple as they seem at first. One would assume that 0W30 has the same viscosity range as 10W40, but this is not the case. The SAE viscosity numbers are not absolute figures, but they refer to a specified interval in the viscosity range.
As most oils are multigrades the figures are double, e.g. 15W50. 15W being the index for the cold viscosity (-18 degr. C / -0.4 degr F) and 50 being the index for warm viscosity (+100 degr. C / +212 degr F) .

Here's the complete list (cSt means centiStokes, an old unit for viscosity):
0W: < 900 cSt,
5W: 900 - 1300 cSt,
10W: 1300 - 2600 cSt,
20W: 2600 - 8500 cSt,
25W: 8500 - 10500 cSt
(@ 255 K / -18 degr. C / -0.4 degr. F)

20: 5.7 - 9.6 cSt,
30: 9.6 - 12.9 cSt,
40: 12.9 - 16.8 cSt,
50: 16.8 - 22.7 cSt
(@ 373 K / 100 degr. C / 212 degr. F)

You can clearly see that the viscosity ranges are not equal in 'width'. 0W30 is cold more than 70 times thicker than warm. 10W40 is more than 130 times thicker. This means that the viscosity ranges of 0W30 and 10W40 are not similar like the SAE-ratings implied.
It may now be clear that the index-numbers are somewhat arbitrarily chosen, 0W would imply to be thinner than 50, but in fact it's the other way around. Maybe the SAE should have chosen numbers that were far apart for cold and warm viscosities to avoid confusion.

Older engines have larger clearances and tolerances. The oil paths are also wider than in modern engines. Using a thin oil could lead to low oil pressure because the oil pump cannot build enough pressure with the thin oil escaping fast. The result is less oil-flow, not more. Thicker oil would not only increase pressure, it is also less prone to seep through cracks and between cylinder and piston rings.

The viscosity says nothing about the quality of the oil. API ratings do.
As the demands from the engine-manufacturers increased, so did the quality of the oil. And if the quality of the oil increased, so could the demands of the engine manufacturers, it works both ways.
Petrol (gas) engines get API ratings beginning with S followed by a letter, the higher this letter, the better the quality:

SA- Straight mineral oil, no additives
SB- Typical non-detergent motor oil
SC- Meets 1964 requirements for car manufacturers' warranty
SD- Meets 1968 requirements for car manufacturers' warranty
SE- Meets 1972 requirements for car manufacturers' warranty
SF- Meets 1980 requirements for car manufacturers' warranty
SG- Meets 1989 requirements for car manufacturers' warranty
SH- Meets 1994 requirements for car manufacturers' warranty
SJ- Meets 1997 requirements for car manufacturers' warranty

Note: if you take a look at the website of the American Petroleum Institute ( you will see different years mentioned than stated here. The difference is that here the years listed are at the beginning of a new rating and the API-source states the year at the end of a rating. E.g. API SF, here: requirements for 1980 models (1980-1988), API-site: 1988 and older (also 1980-1988).

It is not always best to use SJ (latest) in a classic engine because it could contain too many detergents. These detergents are added to the oil to keep debris and soot floating in the oil instead of settling onto surfaces (black sludge). If there already is some degree of settled dirt, the high-dtergency oil could stir up that stuff which creates potential increase in wear.

Mineral vs synthetic
Usually synthetics are too thin to be used in older engines and because of their very high detergency it may not always be beneficial. However, they withstand deterioration best of all oils, and that can only be beneficial. Synthetics also keep their viscosity better during usage than mineral oils.

Steer clear of them. There is a balanced set of additives in the engine oil that may be disturbed by rubbish like Duralube and the likes. If additives do work, they usually only marginally do so. Spend the amount you save by not buying additives to upgrade to a better oil!

Above all
The best oil that's been used too long is worse for the engine than a medium quality oil that has been changed on time.

22,302 Posts
Also interesting, a few years ago, Consumers Union did a big test on motor oil that way to long to get into here but very worthwhile reading and very thorough in their testing. Basically what they stated after the tests was to buy oil based on PRICE, that there were really no differences between top brands and lesser known brands. Also milage between oil changes wasn't that critical.

You can do anything you want to......ONCE!
aka "my 66 coupe"

926 Posts
Thats all fine and dandy, but what viscosity should I be running? Right now I just run the common 10W30 in most everything. My 69 Pontiac 400 called for 10W40 which made me think that 10W40 would be prefered on performance engines. Now if I've got an older car with a lot of miles on it, would I benefit from using something like a 20W50?

66 C-code convertible
66 C-code coupe
65 GTO hardtop
76 Nova hatchback

2,596 Posts
Excellent information, Lennard. It's clear you spent a great deal of time and effort. Thanks.

Glenn Morgan 66 GT Fastback 351w+toploader

1,037 Posts
Yeah, thats my question too... Since my C has the notorious oil system anyhow and its a nice weather car, should I be running a heavier grade than 10w30?
PS-I do run synthetic, got hooked on it in turbo engines because of its low-coking characteristics.


'72 Mach 1 || '90 Talon TSi AWD
"Thats not a leak, my car's marking its territory!"
"If you've done it, it ain't braggin'." -Roy Rogers

Discussion Starter · #6 ·
Excellent post. I personally run Mobil-1 in my daily driver (90 Prelude, a real beater), However, I have been running Regular Mobil 10w-30 in my 2 classics.... I have had great luck. I'm printing your post for future reference.....
Thanks again..

1966 C-Code Convertible, 59 Edsel Villager
Charlotte, NC
New Cars Have No Soul.

283 Posts
I agree that Mobil is the absolute best kind of oil for anything from stock to highly modified. A friend of mine knows one of the engineers for Mobil and he sent him some really interesting information. It included un-biased tests of all the major oils and Mobil was clearly the winner. Too bad I lost all of those papers.

67 FB GT S-code
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