Up front, I'm thinking of dropping this stuff in my other ride. It's a "commuter" with a four banger and rarely gets any heavy foot action, so no "high performance" concerns.

Just inherited a case of Mobil-1 15-W-50. The 4-banger in the commuter ride calls for 10-W-40. If I remember the way those numbers work, that means that the 15-W-50 is the "thicker" of the two. That right?

If that's so, I'm thinking that it would be best to wait until a springtime oil change to drop it in, rather than running a "thicker" oil than the engine calls for in the winter, cold starts and longer to warm up and all that...

Don't want to let a case of the "good stuff" go to waste. Anybody see a problem with using this stuff next summer? (I was thinking of going 3K with it, then change the filter and shoot for another 3K after that.)

wait for the warmer weather. I used to run 20w50 oil in my Hyundai in the summer (for various reason) and besides about 1 second of noise on startup I never had a problem.

Use the stuff, but I would wait for the summer. It pumps better when cold than the ordinary stuff does so your commuter will be fine. You will get slightly lower mileage than if you used synthetic 10W40 but I doubt that the difference is even noticeable.

That oil will be fine in anything but the very coldest weather. In fact, you'd be surprised how easy an engine with syn turns over at sub zero temperatures.
Used in a commuter car, you can go to the max change distance in the owners manual.

dave and xtoy - and we've gone lots further