I agree the zinc is only needed during engine breakin. As for the viscosity of the oil, in my opinion there is absolutely no reason to run anything heavier then a 10-30 oil in a normally sound automobile (big truck different considerations) engine. A lighter weight oil will start flowing easier and get to the bearings quicker. It will also have less oil drag power loss (better economy). A number of years ago, a friend used to run 50 weight racing oil in his 440 drag car. I could not convince him he was throwing power out the window. I used my 170 stock eliminator car as a test bed. Drained the 10-30, and installed straight 30 (same brand), car slowed almost a full tenth of a second average in the 1/4 mile. Drained the 30, and installed the 10-30, and the car picked back up. This was a average of 3 runs with each oil on the same day (9 runs total). He put 10-40 multi oil in his 440, and picked up more then a tenth.
I live in Florida, and tow a 5,500 lb race car/trailer with a 86 Toyota PU with a 22RE 4 cyl engine. It has 331,000 miles with the only engine work being a timing set at 200,000. This motor has had 10-30 in it since new, with no oiling problems. Every auto manufacturer (except the ones recommending synthetic), since about 1960 has recommended 10-30 or 5-30 oil in their engines.
