Re: Oil Synthetic or Not? Gas - What Octane? With or Without Ethanol?

Posted by Owen_Dyneto On 2009/3/6 10:15:27

There has been so much written on this subject of proper motor oils, especially since the requirement for catalytic converters on trucks thus decreasing the zinc/phosphorous additive in the 15W-40 oils, that I hesitate to even try to summarize it.

But just sticking with the topic of low oil pressure (which wasn't the question in the original query, but we seem to have worked our way there anyway), here's my take. If the oil pressure rises to and stabilizes at the correct pressure with a cold engine at idle, you can assume the pump and pressure relief spring are fine. If it rises with a cold engine to the normal oil pressure with a bit of throttle, your oil pressure relief valve spring is Ok, but the pump itself may or may not have some wear. In either of these cases shimming the oil pressure relief valve will not increase the pressure with a hot engine at road speed. The oil pressure is created by the pump and at a constant pump output is dependent on two characteristics, (1) bearing and other clearances, and (2) viscosity of the fluid. So if you use a higher viscosity oil you will increase the pressure because it is more resistant to flow, you are compensating for increased clearances. Though the heavier oil has this greater resistance to flow and cushioning effect, it is by that virtue slower to circulate in the engine and thus less efficient at transferring heat away from bearings, etc.

So, if you need a much heavier oil than was originally specified to get more normal oil pressure at road speeds then you really should drop the pan and check the rod and main bearing clearances; heavier oil will buy you some time, sometimes quite a bit, but it's not the long-term answer.