Depending upon how hard you ru
Depending upon how hard you run this engine, and its current oil consumption rate I will tend to recommend a 5W40 oil.
If you run the snot out of the engine, look for one with an HTHS number above 3.6, if you never see an oil temperature above 250dF, any old (synthetic) 5W40 will be fine (and 80% of xW-30 oils will also be fine). Thicker is not necessarily better, especially with cars that sit for weeks between startups.
Under this HTHS criterion, M1 does not qualify, however RedLine (5W30, 10W30, 5W40, 10W40) and AMSoil (10W40) do.
The lower the first number in xW-y (the x number) the faster oil will get to the vital engine parts after startup (so, lowere is better [in general]). The higher the HTHS number the better the oil can protect when running the snot out of the car/engine (hithger is better but you don't really need anything over 4.0 HTHS unless the only duty your engine sees is on the race track).
My F355 shows lower oil temps and better idle oil pressures after a long hot track session with 5W40 oils than with 15W50 oils. THe lower temp is an indication that these ols are protecting the engine better.
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