51 flathead fluid questions
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Mountain Pass
Joined: Oct 2019
Posts: 196
Likes: 6
From: British Columbia
51 flathead fluid questions
I have new build on a 1951 239 flathead. What fluids should I use
what engine oil. Live at sea level. Summer is in 80s occasionally 90s vehicle stored inside and won’t go out in winter if so not below 40 degrees
what tranny oil. It is a 3 speed manual. Think might. Have been rebuilt but not sure
What differential fluid. Not sure if rebuilt or not.
what engine oil. Live at sea level. Summer is in 80s occasionally 90s vehicle stored inside and won’t go out in winter if so not below 40 degrees
what tranny oil. It is a 3 speed manual. Think might. Have been rebuilt but not sure
What differential fluid. Not sure if rebuilt or not.
Cargo Master
Joined: Aug 2010
Posts: 2,924
Likes: 134
From: Houston
Asking what oil to use is one of the most common questions to spark debate in a car forum. I'll relate my experiences and choices, others will differ.
First, any engine oil you buy today will be significantly better than the best oil available when the vehicle was built. Second, don't worry about zinc for flathead era motors, that was added to oil starting in 1958 to deal with high rpm overhead valve V8s with high valve spring pressures. Don't get worried about old oil specifications.
For engine oil I personally use 15W-40 diesel oil in my old Fords. I made the switch on a friend's recommendation and found the stock 37 Ford sedan had better hot oil pressure than when I used 20W-50 oil. I typically don't run synthetic unless I am willing to deal with oil leaks and the engine is a fresh rebuild. I do run synthetic 15W-40 in my 53 sedan, but I drive that 75 mph on 100F days and like the extra protection when I'm running it hard. Everything else gets Rotella or similar out of a gallon jug.
I find old transmissions and rear axles fell smoother with 85W-140 or straight 140 gear oil.
Just my opinions and experience.
First, any engine oil you buy today will be significantly better than the best oil available when the vehicle was built. Second, don't worry about zinc for flathead era motors, that was added to oil starting in 1958 to deal with high rpm overhead valve V8s with high valve spring pressures. Don't get worried about old oil specifications.
For engine oil I personally use 15W-40 diesel oil in my old Fords. I made the switch on a friend's recommendation and found the stock 37 Ford sedan had better hot oil pressure than when I used 20W-50 oil. I typically don't run synthetic unless I am willing to deal with oil leaks and the engine is a fresh rebuild. I do run synthetic 15W-40 in my 53 sedan, but I drive that 75 mph on 100F days and like the extra protection when I'm running it hard. Everything else gets Rotella or similar out of a gallon jug.
I find old transmissions and rear axles fell smoother with 85W-140 or straight 140 gear oil.
Just my opinions and experience.
Tuned
Joined: Sep 2005
Posts: 250
Likes: 11
We run 10W40 oil in our truck, because in summer it is over 100F. 15W40 would be a good choice as well, but 10W40 would be slightly better at colder temperatures. Does it matter in the end? For us probably not. Both oils perform well at freezing and above which. If it’s freezing we have better vehicles to drive. You will have to determine your use case and look at the temperature charts for each oil and pick your viscosity. For the manual transmission we found that it seems to shift better and less less with the heavier 85-140 oil in it.
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