M1 0W-30 W/M1-110 in my 04' Miata -12K 2-changes per year for warranty
M1 5W-20 W/M1-110 in my '06 Civic -75K changed every 5K because it makes me feel good-commuter car, lots of traffic & heat/ac use
M1 5W-20 W/M1-104 in my '02 Accord -55K -wife's car, driven less than 5k per year, changed annually
M1 10W-30 in my Toro Mower-10 yrs old - changed once a year when I put it away for the winter
M1 5W-30 in my Craftsman Snowblower - 2 years old -changed once a year when I put it away in the spring
Used Amsoil 10W-40AMO in my '66 Corvair because it was the ONLY oil I could find with (advertised) 12-1300ppm zinc & phosphorous
(flat tappet engine) Used WIX 51038 filters-changed once a year.
Have NEVER added oil between changes, never needed to.
Never had an oil-related problem with any of them. Switched from 5W-30 M1 to 0W-30 M1 in my Miata and the lifter noise at startup (less than 2-3 sec) is now non-existent.
I fully realize that with most any of the quality synthetics, PP, German Castrol, Redline, etc, I would have similar results when used with a quality filter, but I can get M1 pretty much anywhere on the road if i need it, The local Advance/Pep Boys/Autozone et al all seem to run it on special, 5qts of M1 & an M1 filter for about $30 on a regular basis, that's $5 a quart and $5 for one of the best filters(IMHO), kinda hard to beat that.