Classic Car Oil - Need Legitimate Suggestions

My manual for my 1950 Cadillac calls for SAE 20, which you can’t really find anymore. Not only that, but the Zinc or ZDDP content in modern oils is apparently much less, and this can be detrimental to an older engine I’m told.

I’m trying to figure what the best oil viscosity is to use for the Cadillac.

On classic car forums I’ve got people chiming in across the board using just about every dang viscosity available in substitute for the straight weight they used to use back in the day. Its a little annoying because I cant find good reasoning to use one or the other. I’ve heard 10w-30, 10w-40, 20w-50, 15w-40. Its like people just choose whatever they feel like!

What would be the safest, most correct oil to use these days for a car that called for SAE 20?

One person mentioned getting Accel oil from walmart because it has an SF rating (for cars built prior to 1988) and has zinc in it. But there was only 10w-40 available. I’ve also heard of using diesel rotella 15w-40 for the zinc content. Seems like people are just choosing the high zinc content oils available and the viscosity is secondary.

So what is it?

this should be interesting…

Haha, I’m not trying to open a can of worms here…its just that one would think, scientifically speaking, there’d be a specific, appropriate modern substitution to use today. People give all kinds of crazy answers across the board. It gets to the point where its almost like “hey as long as its motor oil it doesnt matter”.

I’d go with dino 10w 30 and add a zddp supplement.

Thats currently what I’ve done, but I’m wondering if theres a better viscosity. The engine is old and high mileage. Seems plenty happy with 10w-30.

One thought I had was this: The first number is how it acts when cold, and the second, when hot, correct? So it acts like a 10 weight when cold and acts like a 30 weight when up to operating temp. Shouldn’t I use like maybe a 5w-20 then? I’m worried a 15w-40 or 10w-40 is too thick.

But, I honestly dont know, hence the thread.

I really doubt it’d be too thick on an old, worn engine. I was surprised the manual called for 20 weight.

In 1950, there were no multi-viscosity oils on the market…Most car owners used 20-20W in the winter and 30 weight in the summer. Some automakers recommended #20 oil year round…

Today, 10-30 should be fine…You can boost the zinc with an additive like Bardahl #1.

I I think you’re doing just fine. I think the 30w is appropriate because the engine is older and looser than when new.

“I’d go with dino 10w 30 and add a zddp supplement.”

Or use a 10w30 diesel oil (dual rated) and skip the ZDDP supplementation.

We have come a long ways…

How often does your owners manual say to change the oil Fender?

More fun reading…

I’ll have to double check later, but I’m almost positive it said every 2,000 miles.

There is still straight weight oils out there.
Special order though.

But I’d go for 10W30 too.


10W30 with additives is my vote.
I should point out that there are countless vintage car websites and I’ll bet a lot of them have useful information on this question. Also, the college that I retired from has an automotive program and I know that our library has a selection of books on restoring and maintaining vintage cars. Perhaps your local community college library also has some.

"Caddyman I agree; today we have a vastly larger choice in oil. This engine could use a wide range of oils. If I lived in a warm part ofthe country, a 10W30 with zinc supplement woulk be ideal. Alternately a 15W40 mixed fleet oil (gasdiesel) would do as well.

If I lived in a colder area, a 5W40 synthetic, which has better film strength, would be my choice in the summer. A good 5W30 would do for winter. OP probably does not drive this vehicle much, so I would not worry about excessive engine wear. That 50s 20 grade dino oil all these years at highway speed would have caused most of the wear to date.

Lack of good oils in those years was the major cause of engines needing overhauls at or just beyond 100,000 miles.

If the engine has not been rebuilt and you don’t drive in the winter, then I’d recommend 30HD. I think your recommended oil change interval was 1500 miles. As I recall that was the recommended intervals on my 55 Chevy and 57 Olds, but oil and oil filters weren’t very good back then.

If you drive in winter, you should continue using 10w30 as it is working for you now. One way to tell if you have gone too thin is that your oil light will come on at idle so if you try 5w20 or 5w30 and the oil light comes on or flickers at idle, then go immediately to a heaver oil.

I see, if you dig, you can still get 10w/30 with an SL service rating…The Shell Rotella line has it, along with many other “Rotella” varients…API-SL was the last certification that contained the full dose of Zinc extreme pressure additive…This oil is also rated for certain diesel engines making it a dual-use product…Other labels that promise a full dose of zinc, “Motorcycle Oil” and “Racing Oil”…

While the shelf-space available for a new line of motor oil is limited, I can see where a “Classic Car Oil” would have a ready market… All that is needed would be the API-SL formulation. The label could state “Full Zinc Protection For pre-1975 Cars” Then, in smaller print, “This product may reduce the life of catalytic converters”…

Exxon, are you listening? They’ve got a ton of various products to confuse anyway like 5000, 7500 etc. Why not a classic car oil in maybe black containers?

I would hazard a guess that your Cadillac owner’s manual recommended 20 weight oil and the oil back then probably was non-detergent oil. There used to be a problem switching cars to detergent oils–it caused the oil consumption to increase dramatically. However, your Cadillac has been around so long that it probably did get a crankcase of detergent oil sometime in its life Check the manual carefully as to whether or not it required detergent oil If it recommended oil labeled for “Service SA” or for “Service SB”, the recommendation was before detergent oil. However, it will do just fine on 10W-30.

One thought I had was this: The first number is how it acts when cold, and the second, when hot, correct? So it acts like a 10 weight when cold and acts like a 30 weight when up to operating temp. Shouldn't I use like maybe a 5w-20 then? I'm worried a 15w-40 or 10w-40 is too thick.

That’s correct, however, the oil does not get “thicker” as it heats up. Cold 5W oil actually flows more slowly than hot 40. Think of 5W-20 as 20 weight oil that doesn’t turn to molasses when the temperature is freezing cold.

When they started to use hydraulic valve lifters, these engines needed detergent oil…The Cadillac and Oldsmobile’s, these cars were among the first to use hydraulic lifters, eliminating valve adjustments…These new V-8’s appeared in 1949…