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Non-detergent oil for a 1939 Pontiac stock 6 cyl. engine

I have been unable to get a conclusive answer on what oil is compatible with this engine. The original specification says to use 20 wt NON-Detergent oil. I need to change the oil, and would appreciate any help available. Thank you.

Scerto5 - I don’t have an answer for you but I’m anxiously waiting for anyone who does. I have an original '32 Chevy with a freshly rebuilt Blue Flame six. I even contacted Mobile Oil and was thrilled when they replied to me. Unfortunately, the reply was “we don’t know.” They explained that they don’t do development on antique engines. They went on to explain that if I could supply them with the required oil specifications they would tell me which oil they recommend. I am patiently awaiting responses to your question.

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I like your Chevy.
I have heard that Shell Rotella 15w-40 is supposed to be the answer for a rebuilt vintage engine. It has the necessary zinc content and is made for classic cars. My issue with it is the engine in my Pontiac is bone stock and it needs 20w non detergent. I found a 30w non detergent oil, but I’m not sure that it won’t be too heavy and I can’t find the zinc spec. I also found a vintage car oil made in England, again I just don’t trust that it will be compatible with my engine.1939%20Pontiac%20woodie

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I don’t have a technical background in oil, but was always told by experienced old car mechanics that you can go from detergent to non-detergent, but NEVER the other way around. If your engine was recently rebuilt a modern multi-viscosity detergent oil with the needed ZDDP (zinc) content is fine. If it’s been running on non-detergent then you need to stay with that.
If you must use non-detergent oil, 20w sounds appropriate, and you can’t find it from any specialist sources you might have better luck with rural/agricultural supply outlets… i.e. Tractor Supply, Bomgaar’s are common in the Midwest.

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@Jim-R is correct to my understanding as well, farm equipment typically runs non-detergent (John Deere dealers have been a source for me in the past, but can be pricey.) 20 weight is likely thin for implement engines though, I would imagine they run thick (30 and up), but it never hurt to make a few phone calls to check.

Personally, I would not worry about switching to 30 weight over 20 weight. Add a zinc additive to get the protection you desire, and enjoy driving the car. That 10 weight increase is not extreme, and you could always switch back to 20 weight non-detergent if you don’t like it or find some adverse affects.

Thanks, I’ll look into that. It sounds as if the rural supply might be a good place to find the 20w.

I do have the additive and I’m waiting to hear from the manufacturer with the specs.
I’m also going to have the current oil analyzed, as that might tell me exactly what is working now.
Thanks for your reply.

Again…and I’m with Kyle, I don’t think finding a straight weight oil is any priority. It’s the detergent vs. non-detergent that’s important. A 10w-20 or 10w-30 would actually offer superior protection on cold starts over straight-weight 20.
Apparently there are a lot of old tractors out there that still require non-detergent. Up until he finally sold it a few years ago that’s where my 91 yr old dad still found non-detergent for his 51 Ford 8N. :sunglasses:

I have found a 20w non-detergent engine oil, made for automotive use.
I am also going to have the existing oil analyzed to be on the safe side.
Both are fairly inexpensive and should give the results I’m looking for.
I will post the results, soon.
Thanks for the help