John, Yes, a 30 year old document. I've been running the 15-50 Mobil 1 for quite some time now, quite happy with it. Porsche ws one of the "holdouts", suggesting single weight oils long after other car makers had switched to multi weights. I think the answer was something about air cooled engines being different? And yes, Mobil 1 is at Wal-Mart now, 5 quart jugs, $17.88. Not a lot of 15-50 left at my local store though. ;)

I may be mistaken, but I believe motorcycle oils contain special additives/friction modifiers/whatevers because the oil is used by both the motor and the transmission. I can't imagine the additives would harm anything as they are obviously safe for the motorcycle motor. Question is, are the additives providing any added benefit if used in the 911?

Warren's cost point is a good one. I'd hope the added cost of the motorcycle oil would provide you with something more than what you get from the regular high dollar Mobil 1. I am a Mobil 1 user as well. Why not? I only change the oil before winter storage and before it's time to roll in the spring. I just dump some Exxon, Castrol, Mobil or whatever in there for storage, reusing the filter until spring.