The viscosity of your engine oil has been determined by the manufacturer itself, The design of the oil pump, the clearances of the bearings, piston skirt to wall etc. etc. all are taken into consideration. Using a way too thick oil can show you good oil pressure but this also means that you have a limited volume of oil flowing "through" stuff - (this is where the oil is being used as coolant)
too thick an oil can literally tear apart the film its making (unless your oil is blended such - e.g. european spec BMW LL or MB spec LL oils) - even then you are limited on the top end of the viscosity range, e.g. BMW 10W60 oil is a killer if used in their regular passenger car engines other than their M (motorsport) range - they literally specify against it.
Honda, Toyota, Mazda etc. have specced a very light oil to be used in their engines (new camry is filled with 0W16 oil) - because they have built it so, they have a slightly larger sump to aid in cooling and slightly tighter clearances - this makes for most power delivered to the driveline without wasting it. (same reason BMW now use variable volume oil pumps and electric water pumps now). On the regular I see fleets of these cars that are run like hired donkeys and one engine oil charge is replaced at 10,000 miles - the engines are top notch fantastic. I wonder why they dont simply die running in south texas or the texas panhandle. - If you want a solid comparison, lets to to miami - where the humidity is like breathing underwater, fuel quality is so poor that it has twigs in it and weather so hot that you can die within 10 minutes under the sun. Old chevy and fords here run cheap 5w30 (chevy/ford quality is LOL)
if you are so worried about the viscosity - why dont you use a thicker gear oil in your automatic transmission or power steering etc?
