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Here is the bit of a quandary in increasing the oil viscocity such that the oil has a higher HTHS:
The dynamic viscocity (DV) and kinematic viscocity (KV) are generally related in Newtonian fluids (such as oil) by the formula DV = KV x SG, where SG is the specific gravity of the fluid.
The KV is measured and quoted as a flow rate through a standardised capillary tube between an upper and lower fluid reservoir in a calibrated apparatus. This means that a quoted DV at 150°C for one oil that is 25% more viscous than another oil would cause 25% more resistance to flow at a given temperature.
At high RPMs on an engine that is warmed up, you want to be at the maximum oil pressure (on the S54 this is regulated to 4.0 bar). What this means is that because the oil is incompressible and the design engineers will expect maximum oil pressure to be reached before maximum revs with the designated oil, a 25% more viscous oil will have 25% less flow when in the expected oil pressure relief valve bypass mode at maximum revs.
25% less oil flow will lead to less cooling and hotter running in areas of the engine such as the underside of the pistons. The lower flow through the whole of the oiling system may mean cooler oil at the oil cooler (the oil spends more time being cooled due to lower flow rate), but overall the effect will be a change in temperature gradients, with the hotter parts getting hotter and the cooler parts getting cooler. The big-ends may run cooler, but the little-ends (wrist pins) may run hotter to the extent that the thicker oil may run a lot thinner there, thereby changing both the flow and temperature gradients further. Depending where the temperature sensor is and the overall change in flow and temperature characteristics, the oil gauge could indicate the engine is running cooler or hotter or the same.
The only certainty is that the engine oiling characteristics will be different than the design engineers originally measured.
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