i ran synthetic blend in mine for a while, but it just seemed to burn it up really quick (almost 1 quart every thousand miles and only 75-80K on the engine). no leaks, no smoke, don't know where it went.:shrug
now i just use regular old 20w50, no probs, except my shitty fram filter blew its gasket this weekend, 300 miles from home.:banghead
i'm thinking of useing it in my rebuild after talking to one of my instructors whose built a bunch of engines says every 3,000 he spins on a new filter and tops off. recently took apart one of them and the thing was spotless.
The guy I got my truck from ran Mobil 1 in it, and so I have used it for over 3 years now as well. Seems to work great for me, and it seems to stay cleaner longer.. Usually change about every 3500 or so...