L Head Oil Pressure

I rebuilt the L head my M38. When not hot or at higher RPMs it has plenty of oil pressure. After driving a few miles at an idle it has pretty low oil pressure 5-10 lbs. What pressure are your L heads running at a warm idle? Also what oil are you using? Im still running 20WT brake in oil.

 

Probably ok, might try changing oil to a slightly heaver grade 10w30 or 40 and see if it improves. 20 weight is quite thin oil unless the air temp is below freezing.

 

The L-head in my 47 hovers around 20 lbs at idle, between 40-60 lbs at higher RPMs. About the same with the F-head in my 67.

I don't use multi viscosity oil in these engines, just straight 30wt. In a high mileage engine I might run 40wt.

 

The old rule-of-thumb is 10 psi for every 1000 RPM.

Oil pressure does not matter as long as the oil film in the bearing is not squeezed out and there is no bearing contact with the journal. This also depends on how you drive. Fast acceleration pushes harder on the pistons and thereby on the oil film.

Multivis changes the temperature characteristics of the oil, but should not be much thinner than single-weight. 10W30 for example, is 10 winter or it behaves like 10 weight at -13F. This oil is also required to behave like 30 weight at 212F. Viscosity is still going to change a lot with temperature, just not as much. As I understand it, this is a 10 weight base oil with compounds added to make it more like 30 weight at high temperature. Supposedly these enhancing compounds break down as the oil ages.

The main advantage to the multivis oil is lower cranking force when it's really cold. Once the engine is started, you should be able to maintain a higher operating temperature. I understand that the thinner base oil causes less drag in the bearings and improves fuel economy.

These engines originally called for single weight, and that should also work fine. I would also expect Jeep to recommend a change in viscosity based on the air temperatures where you live.

 

We rarely see really cold weather in my part of the country, and when it does get really cold my Jeeps will stay in the barn.

 

After rebuilding I am running a 5w20 Lucas brake in oil. Reading the warning below and the TM1804A stating to start up with 10wt. I do not want to run 30 wt. just yet. With all the time and money invested I don't want to do any damage running a heaver oil. That said I don't want to cause damage running such a light oil. I know I am moving oil. What Tim is saying makes me feel more comfortable about the low pressure at a hot idle. At higher rpms I am running around 40 psi.
Thanks for the help