Reduced model of the oil flow system
What you see below is a oil tank, an oil pump, a pressure chamber with two outlets: one to the engine and one to reverse flow path. The pump, which is connected to crank speed only, conveys the oil from the tank into the engine oil system as a main function. The the pressure of the oil system can be controlled, a too high pressure can disturb the function of the tribologic system, a relief valve is placed in that chamber. It opens at a certain oil pressure, balanced by the spring. As the spring delivers a force, F = spring rate * compression height difference, it is clear the pressure must increase to further open that valve. The distance from closed to fully open is around 3 mm, while around further 3 in the beginning are just from closed to slightest open. It is no linear opening at all.
For a model simplification we can say there is a opening pressure and a full open pressure which are different, but their distance is quite close together.
Scenario tight bearing:
The main pressure drop happens at the bearings obviously. The smaller the clearances the higher the restriction. The oil flow sees a restriction and immediately pressure rises as the positive displacement pump has no mercy, all must go off the pump into the pressure chamber. Here the oil follows the engine oil path till the point the relief spring give some opening to that valve (drawing double triangle = valve system). Thence a part of the oil flow, so much that the oil pressure get lowered to the release level by bypassing the engine oil path into the reverse feed (= suction line, see thick point connection). The reverse path is directly to the pump connected, maybe the drawing does not apply to locations, just on process rows for abstraction reasons.
If pump speed increases further more reverse flow is created while no higher oil flux goes into the engine oil path. Now you understand why clearance design and oil weight consequently define how much oil goes through the engine. If the pressure further increases, less oil sucking flow will go into the pump as the efficiency goes down and more is just pumped around the bypass and only a slight little more goes into the increase of engine oil path. The system is sort of saturated, efficiency controlled. This safes the engine integrity.
Scenario loose or right bearing:
At the releasing oil pressure more oil flow is supplied to the bearing, which gives it a better tribological integrity and more tolerance on higher engine speeds and every extra force on the bearings. The engine speed at which the relief valve opens is at higher engine speeds. But also here: there is a point where the system stalls and further increase of oil pressure just leads to tiny increases of oil mass flow through the engine and any increase goes into bypass flow increase until the system is saturated.
What is the outcome:
- A remark here to the consequences: the valvetrain get more oil. The oil will be cooler and less peak temperature will be seen on hot spots like the small end rod pin or journal bearings. The viscosity design is a bit easier, as one can play with pump speed gear ratio, engine oil temperature and so on.
- Same is valid for viscosity. A lower viscosity corresponds to loose or right bearing clearance and a higher to more tight bearing clearance. Thence for the sake of a higher oil film height less oil film is pumped through the engine. Is this more safe? It depends, but for sure a 5W50 is less safe then a 10W40. It might be worth testing to see if a 5W30 is better fitting the system as an 10W40 oil. For sure a 0W20 is not improving over a 5W30, as the oil film height is quite engine speed sensitive with those, especially at hot start up and low engine speeds.
The K20C1 comes with a highly Molybdenum containing 0W20 oil but has the same clearance recommendations as the K20A2 engine. That means fabrication tolerances has to be decreased to make that system working. I have contact to a oil passionated K20C1 Green Hell driver and he reports quite good wear results with 0W20 on the Gree Hell use. But the K20C1 has a higher ECT level as the K20A-series, no one knows why
. Did I already elaborate why I don't like boosted engines 🤣?! Anyway, a 0W20 oil approach can be run on the K-series, but lower maintenance intervals and only good additive packages and base oils, no aftermarket oil from Penzoil, Castrol, ...I recommend Amsoil SS, Ravenol VSW or RSP, Quarker State FS, as they have shown good results even down to 0W20 levels.