Technical High flathead oil pressure

quit worrying about it, worry if you lose oil pressure

 

This is in layman's terms the way it was explained to me.....
15W40
15W means it's basically a 15 weight oil. It pours and flows like a 15 weight oil.

40 This is the crushability. Meaning, the viscosity between the bearings is like a 40 weight oil.
So it flows like 15wt with the crushability like 40wt. Thus, you'll have higher pressure like heavier oil.

The standard original recommendations for flatheads was SAE 20. SAE 30 for the more extreme (hot climates) SAE 10 for extreme (cold climates) or even cutting/thinning the oil with a small amount of Kerosene in extreme cold. So 10W30 would seem ideal.
The Multi rated oils solved a lot of the problems related to straight weight SAE oil.

Some will run the 20W50 VR1 racing oil. (the zinc debate) On a worn engine this will shoot the pressure up but kind of like Tubman says, I don't know if this is any real benefit. Personally I think it makes the operator "feel better" running a "thick " oil.

With that said I have a flathead engine now (in storage) that runs very low pressure on 10W30 at idle once it's hot or ff the oil has any age. The pressure will come up at RPM and honestly you could use the oil gauge like a speedometer. It ran 10 or less at idle and up to 50 60lbs at highway speed. When it was good and hot it ran substantially less 7 to 45-50 depending on speed. Since it idled at such a low RPM I felt it was OK and met the 10ld per 1000 rpm rule. This gets into the (flathead oil pressure debate) Like zinc and hardened seats there's differing opinions.

Flatheads are old designs and I think you'll find the oil will thin the longer it's run. Gasoline and the products of combustion will thin it over time. I don't believe it's enough hurt anything as long as the pressure is good.

 

It's always interesting to read these threads on oil and see peoples thoughts and perspectives. As a credentialed Certified Lubrication Spe******t with the Society of Tribologists and Lubrication Engineers, employed as a Lubrication Engineer for over 20 years, I have some professional background on the topic.

Regarding pressures, excessively high or low, there can be problems with either one. If I had to choose between too high, or too low, I'd choose too high of course; but best is the proper pressure for the application. Yes too high can be problematic, depending on the situation, but if the oil flow is such that the oil pressure constantly has the pressure relief valve open, that can cause heating of the oil (leading to oxidation/shortened oil life), and it can cause shearing of viscosity index improvers in the oil, resulting in a loss of viscosity. So yes, too high can be problematic.

Regarding diesel engine oil vs gasoline engine oil, and the suitability of using one in the other, there is always a lot of misinformation about this too. Older engine oil specifications, and older formulations to meet those old specs (as well as higher levels of sulfur in diesel fuel) meant that diesel engine oils were not great to use in gasoline engines, because they could lead to increased piston deposits. Diesel engine oils had higher levels of sulfated ash, which was needed to deal with higher levels of sulfur in the fuel. These days sulfur in diesel fuel is limited to 50ppm. It used to be 10x that level. So with Ultra Low Sulfur Diesel the need for higher ash levels in the lubricating oil is reduced, and there is now a limit of 1% sulfated ash in today's diesel engine oils. So the concerns about piston deposits really aren't a problem any longer. Some folks claim that the higher levels of detergents in diesel engine oils is too much for gasoline engines. I call BS on that, please show me the SAE papers that say that. IMO modern diesel engine oils are a fine replacement for older pre-smog equipment gasoline engines of the HAMB era. And as someone else above pointed out, the viscosity grades are just about right too. I mentioned above that 10W-30 is probably a better vis grade for an old flathead, but 15W-40 is probably very good for newer OHV engines that typically ran SAE 40 straight grades or 10W-40 multi vis grades. For some climate zones you might want to use 10W-30 for winter operation, and 15W-40 for summer operation, it really depends on your climate and the seasonal temperature swings, and also your engine and how loose or tight it is.

Anyway, it looks like the original problem for this thread is resolved. I don't know why or how, but as long as the pressure is normal, what the heck, go with it I guess.

 

What's the operating temperature (thermostat) you use? That, and oil temperature, has a huge effect on oil pressure.

Cold oil is thick oil. I've got the OHV version of a flathead engine (the Rambler 195.6) that the builder clearanced for 20W-50, he said. I run 190F coolant. The oil pump has a 60 psi relief valve. I've got a big oil cooler with a computer controlled fan on it (modified pump). I stuck a temperature sensor in the oil pan.

When cold, idle is 40 psi, climbs to 75 psi at ~ 2500, so I baby it until oil warms up, 100F oil temperature, and keep my foot out of it until oil is 130F. I drive it hard in the summer, with a bunch of sports car folk, desert mountains. Warmed up idle is 25 psi, 3000 is 55 - 60 psi, "normal". Oil temp runs around coolant, never more than 230F.

In the winter I've taken to running 10W-30, though warmed up it thins and warmed-up high rpm pressure drops to 52 - 55 psi. But 10W30 is cheaper and since it's an open car I don't drive it winter like I do summer.

The point is, temperature and temp control matters.