stangman..straight 30w oil??? where do you live man? i hope its hot there all the time.
slugga, i agree with hissin50, the detergents in the synthetic help clean any spots where there is sludge which can cause leaks, or make a small leak worse. the high milage oil has additives in it designed for the wear that has been done in a high milage car, as for the weight, depends on where you live and whether you drive it in winter. if you get cold winters, well below freezing, dont put straight 30w in it, use a 5-30 or 10-30 summer you can get away with a heavier oil like 20-50 but personally i wouldnt where i live.
there is no difference in the 5-30 and 10-30 at operating temp, they are both 30w then, but if its below freezing the 5w flows faster up into the engine at cold temps, less wear. 20-50 in freezing temp is like molassis.
if you are in the southern areas where there is little cold you can do it though.