When I can afford it, Mobile1, 15W50 summer, 10W40 winter. When I can't, some lower cost syn. I,ve used Honda HP-4 semi-syn and liked it but it is too expensive. As far as why, It is really hard to know one is better than another if it doesn't blow up or break, but I have a friend that used to spin wrenches for Skip Barber when they had the Saab Turbo F3(I think) series. He said when they switched to Mobile1 engine problems dropped noticeably. They also doubled the run time on mobile1. When I raced Vintage bikes, the guy I rode for had Older British twins and triples that were really hard on the valve train and cranks. stock they would break if they saw 6k more than every once in a while. I shifted his twins at 8200-8400 RPM (or whatever chip he would let me have that weekend) every gear, every lap and we never had valve train problems and only lost one engine in two seasons. It said a lot for his engine and trans building skills, but he said the oil made a big difference in slowing valve train wear and breakage. I figure if it works that well under those stressings, it should be great in a modern engine.
About the only difference I can tell from the seat is shifting. I used an old SOHC 750 Honda as a test mule for oil at one time. It had a pretty clunky trans. The clutch would slip with full synthetic (never had that problem on a newer bike) so I tried mineral oils. The slickest I used was Valvoline racing but it went to shit at 800 miles. Pennzoil was the next best that stayed slippery for the 2k I ran it and didn't cost much.
Honda HP4 semi syn was about as good as the Valvoline racing oil and stayed that way until I changed it. I tried about 6-8 different oils and Pennzoil remained the best non-syn I tried. The worst was...... Castrol GTX. I know what the ads say, but the toes knows.
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