Simplest way to lower viscosity? Mobil 15w50

Since you're using Mobil 1 15w50 the easiest would be to mix in some of their 5w50 to help lower the winter rating. I've found in our diesel farm tractors that switching over to a full synthetic or even a blend really helps them crank over a lot easier during the winter. Do you have a block heater that you can plug in?

You can also try adding a quart of MMO as they recommend it during the winter months for easier starting. I've used it in my Honda Civic and GMC w/ 5.3, supposedly works great for freeing stuck lifters and rings but I haven't experienced those issues but figured a bottle every so often can't really hurt either other than my wallet. It's not something I use all the time either, typically every 3rd or 4th oil change I'll use it.
If you don't want to get a thinner oil and change it, which IMO would be the best thing, MMO will thin it. The problem is if the sump is full you have to drain some oil, to replace with MMO. That can be a PITA, the other issue is you won't know how much you actually thinned the oil. If I had 15W50 in the sump going into the winter, I'd be changing my oil, no matter how many miles were on it. If the filter had life left, from short service I'd leave it alone.
The big block in my Chevy truck is enjoying Mobil 1 15W50. The oil has maybe 2000 miles on it. These 496/8.1L engines are known for oil consumption. This oil with a lower temp thermostat stopped consumption totally. I thought I would have still lost a quart or so by now and I could have topped off with a 0W20 to help thin it out for winter, but no.

Kind of seems like a waste to drain out the oil. At what temp is 15w50 going to be too thick? I'd guess cold starts under 30 degrees aren't ideal.

Any acceptable ways to thin it down some? Use the remote start more? Ha.
Add some Marvel Mystery Oil.
I've had trouble starting an engine with 20w50 below 10f. Coldest ever starting the Borman6 with 10w40 was 17 below. The last 528e got a pint of MMO for Christmas in its final yrs :cool: