1941 Pontiac L239 Engine

All,

What oil is best to be used in my rebuilt 1941 Pontiac L239 engine. I am just about ready to fire it up and need some help. Reading all of the information on here about Detergent vs Non-Detergent oil is a little confusing and the fact that my engine does not use a more traditional style of filter system makes me want to get this right the first time. Any suggestions are more than welcome.

Thanks

Jason

If it was mine I would use a good name brand 10W30. Once it was broken in I would try synthetic if I thought the seals would hold it.

I know what you mean about the filter in the oil pan. I would be tempted to supplement that with an external bypass filter like a Frantz for in depth filtration.

Jason, the cheapest oil you can buy is better than what was used in the '40s/'50s. As Rusty said, any good name brand 10w/30 will be fine.

Ben

If it's a brand new, totally rebuilt engine, then the detergent or non-detergent question is meaningless. The problem on an older engine is that detergent oil may break down some of the crud that's hanging around inside the engine, causing issues. You can use either one in your clean engine with no issues. Since I mostly drive my cars in warmer months, I prefer a straight 30 weight oil.

The problem with running something like 10W30 is that, when cold (engine temperature, even in summer), it's a 10W oil, and that's what you're running your car with until the engine heats up. That's fine on new cars, but I don't like the idea of running an older engine with 10W oil for any time at all. Once it heats up it "acts" like a 30W oil.

Multigrade oils have additives that make them thin out less when they get hot. They still thin out a bit. There is no down side to using them.

In the 30s, before multigrade oils were invented, some expensive cars like Pierce Arrow had oil cooling systems that kept the oil at a constant temperature. They recommended 20W oil year round.

30 oil was usually recommended for hot temps like over 70 degrees.

Jason, the cheapest oil you can buy is better than what was used in the '40s/'50s. As Rusty said, any good name brand 10w/30 will be fine.

Ben

Agree

The 10W part of the oil name means it acts like a 10 weight when cold. It thus flows better than 30 weight oil when the engine is cold and there is less wear at startup. The -30 part refers to the viscosity when hot. If you use a straight 30 weight, it is higher viscosity (i.e. it is thicker) and less effective at startup. You can see graphs of viscosity versus temperature in various places, for example on Richard Widman's paper on this forum, or at

http://www.widman.biz/Corvair/English/Links/Oil.html.

Presumably the engine has flat tappets. You might like to use an oil with "enough" zinc to extend their life. Use a CI (diesel) 10W30. CJ is OK but has a bit less zinc.

In a flathead the valve gear is lightly loaded and usually does not need zinc additive. It is the pushrod OHV engines, especially high performance models, that suffer from lack of zinc.

All,

What oil is best to be used in my rebuilt 1941 Pontiac L239 engine. I am just about ready to fire it up and need some help. Reading all of the information on here about Detergent vs Non-Detergent oil is a little confusing and the fact that my engine does not use a more traditional style of filter system makes me want to get this right the first time. Any suggestions are more than welcome.

Thanks

Jason

I'm restoring a 46 Roadmaster. This is the oil I will be using in my 320. You need the zinc to lubricate the metal surfaces on start up. It's just like the lead that was added to gasoline in the old days to lube the valve guides & SEATS.

I was under the impression that lead in fuel was to lubricate the valve SEATs. Without lead, tiny nodules build up on the face of the valve (esp. exhaust) and as it rotates (caused by using spiral valve springs) it grinds away the seat. Hence hardened valve seats are installed now in overhauls.

Zinc was added in the fifties to protect heavily loaded cam and lifter surfaces in OHV engines with hot cams and stiff springs. It was not needed in flatheads and it is not needed in OHC engines because neither have pushrods and neither need such strong valve springs.

Zinc was added in the fifties to protect heavily loaded cam and lifter surfaces in OHV engines with hot cams and stiff springs. It was not needed in flatheads and it is not needed in OHC engines because neither have pushrods and neither need such strong valve springs.

There's lots of research on the net about proper oil and using unleaded regular gas in our old cars, so I could go on and on. I'm going with the zinc-added oils, and since I don't rod the old motors, am using regular, unleaded gas. Good luck to you all.