[Smiley913]

To start off I would like to introduce myself. I am a chemical engineer. I have a good understanding of hydraulic (how fluid flow) and pumps.

I want bring up the point using 0w40 to minimize bearing wear. The idea is that thinner oil will have more of flow compared to thicker oil. Therefore, it will provide better protection for daily driving.

Because I am not an automotive engineer, I have no idea what protects an engine better. However as a chemical engineer, I can say the statement of using thinner oil will result more flow in a car engine is incorrect. The reason is because the oil pumps in cars are positive displacement pumps. The basic idea of a positive displacement pumps is it will always move the designed amount of volume regardless of discharge pressure. The reality is pump discharge pressure does have a small effect on flow rate due to pump imperfections. But for this discussion, I will limit it to only theoretically how positive displacement pumps operate and not reality.

Based on positive displacement pump principle, the amount of engine oil that goes through oil pump is the same regardless of oil viscosity (0w-40 vs 10w-10) as long as the pump has enough power. Before anyone say “does that mean I can use 10w-100 because viscosity doesn’t matter in positive displacement pump,” the answer to that question is both yes and no. If the pump has enough power and the pump components are strong enough, the theoretical flow rate of 10w-100 and 0w-40 is the same through a positive displacement pump. However, the oil pump in cars are limited by timing belt (I think that’s the components that connect the oil pump to the engine) and pump components as they are not strong enough to handle all of an engine’s power. That is why relieve valves are installed to prevent overloading the pump. So if you use too thick of oil, the pump discharge pressure would be high causing the relieve valve to open. When the relieve valve opens, some flow is diverted back to the oil pan without going to the engine components. To summarize, if too thick of oil is used and the oil pump has enough power, the flow rate through the positive displacement pump would be the same. However, the oil going to your engine components will reduce dramatically as most flow is diverted directly back to the oil pan through the relieve valve.

If S65 oil system is designed for 10w60, I don’t see how using thinner oil will result in more flow.

Positive displacement pump:
Thick oil = higher discharge pressure = flow unchanged
Thinner oil = lower discharge pressure = flow unchanged