Me and my freind both just changed to mobil1 full synthetic about a month ago.

I have a 94' accord with 128K on it.
He has a 87' 325 with 160K on it.

No leaks or problems on either, it helps alot with the cold starts thats for sure, probably cut the turn over time in half, and its in the 20's in the mornings here

And no, once you go synthetic you CAN go back. If your worried about it, you can always swtich back, runn 500 miles, and change again to flush it all out. Thats my thoughts on it anyways

But i would recomend the switch i am happy with mine, and i read somwhere that if you swith to a synthetic in your motor and manual tranny, you gain 2-3 WHP...Always a plus