[STORMINORMAN]

I would recommend staying within the same manufacturer (i.e., mixing M1 with other Mobil oils) and staying "close" as far as API classifications are concerned (i.e., OK to mix SL & SM grades, a bit chancier to try to get max performance out of SF mixed with SM: too much of a swing, IMHO). OK to mix different weights, too, but you don't get a "new, improved weight" in your engine: what you do get is a proportunate mix of the different molecules, which may not be a bad thing.

Not that this kind of mixture will actually hurt your engine: just that "you can't make a silk purse out of a sow's ear". The SF oil will not magically become SL (or SM) by adding some M1. The modern API grades all say they are backwards compatible in their specs. Just don't ever use oil that pre-dates the spec for whatever engine you are running. No SA non-detergent from Autozone in your SVX, please!

I have had no problems with a winter mix of 50/50 Delo 15W-40 and Cheveron Supreme 10W-30 in one of my older Mercedes V8's. back in the "old days" my dealer used to put a 50/50 mix of synthetic & dino 10W-30 in my SVX for no extra charge as part of my "lube for life" program.

That was pretty sweet: 1/2 synthetic OCI every 3K for $0.00.

Cheers!