What grade engine oil to use on aluminum M119?
I recently dicussed engine oils and their viscosity with a local mechanic.
He has this theory that aluminum block engines should always be serviced a grade lower in viscosity then cast iron no matter what the ambient temperatures. I live in Hawaii and ambient temperatures never go below 0 degrees Celcius, so I have always been using Penzoil 20W-50 for my cars. ( 85 190E 2.3, 88 300E, 92 400E).This guy suggested I use 10W-40 instead for the aluminum block. He claims that with the higher viscosity of the 20W-50 there'll be no oil around the rings to protect the cylinder walls from being shaved by them at start up, resulting in compression loss and ultimately 0 compression on the cylinders.
Any thoughts on that? What about synthetic oils?