I've used Mobil-1 10W30 in mine since new. I've also used Mobil-1 5W30 on wife's Honda since new. Both are the manufacturer's recommended weights.

I still use dino-juice in the '86 Porsche and '88 Merc. The Porsche still doesn't use oil. I use whatever falls readily to hand for the Merc, it could be anything from 10W30 to 20W50 Castrol. I'm not brave enough to change from dino-juice to synthetic late in a car's life, I've heard stories of doing this and then sprouting oil leaks like a colander. I have done this on my lawn equipment with no leaks. Yet.

-Punxsutawney Phil

'98 V70T5M, 120k mi., iPd stabilizer bars, Volvo strut tower brace and skidplate, Valentine-1, Mobil-1 always, e-code headlight lenses, Dunlop SP5000s

'86 944NA (that OTHER 944) 58k mi., factory HD sway bars and Fuchs forged wheels, Valentine-1. e-code Hellas, Yokohama AVS ES100s