i use mobil 1 syn 10w-30 for my gsr in summer, 5w-30 syn in winter.

that "once you use synthetic you can't go back!" urban legend needs to DIE! the only reason switching back and forth isn't good is because the synthetic is thinner, so it'll eventually "push" the old oil through seals it created. then when going back to dino, it might leak for a while.

but going from syn -> dino is not a problem.