all oils have different thermal characteristics. Their SAE rating is based on several tests and their viscosity rating is based on these tests.
Going form a 50 to a 40 means your oil is thinner at the same operating temp, this will account for your lower oil pressure. And depending on the oils thermal capacity and ability to remove heat from components will alter engine and oil temps.
Mobil 1 15w50 has a good viscosity until you reach the higher temps like 110-120 where its thermal load is too much and it looses alot of viscosity, a poor VI. So i dont use it anymore, high cost and poor performance.
For a good and easily available oil, just pop along to your ren dealer and pickup some Elf COmpetition 10w40 semi synth, its got great thermal characteristics and holds up well on track and motorway. If thats too thin, try a 5w50 or 15w50..........and if you really got the cash, Torco MPZ oil from the states is fantastic.
