Anybody got any comments on whether I should use a 5w50 (Castrol Syntec) instead of the 20w50 the engine manufacturer recommends. I don't suppose I should say what I'm putting it in because everybody seems to clam up and won't talk much about motorcycles, but it's for my '00 Harley Twin Cam 88. I'm going to do a dose of Auto RX so I needed a non-synthetic (Frank said Group III was OK) to run for only 750 miles and had a freebie from Sears ($20 gift card because they didn't change my tire in an hour). The Castrol 5w50 was the only 50 weight they had that was diesel rated (what Harley recommends if their oil isn't used). Anybody got any viscosity numbers on the 5w50? Castrol doesn't put MSDS's on the web and there are no VOA's or UOA's on this stuff, but in the Why-buy-any-other-synthetic-than-Mobil-1 thread someone suggested that the 5w50 might hold up better than M1 15w50 (which I am considering using later on). But since I'm only going to run it 750 miles, should I care just about the lower limits of protection rather than about the upper limits? BYW, the ambient temperatures here in Alabama are still in the 70's at night but it can get well into the 90's during the day.