Changed the oil today Mom bought it for me...:P
 

i asked her to get mobile 1 5w-30 and a toughguard oil filter and she wrote it down in her mileage book...ok. She went over to fleet farm but forgot to take her mileage book....so she told and auto tech what car it was and that i wanted 5w30. He gave her 5w-30 but it was truck/suv heavy service!!!! He must have been tired or something. He also gave her the extendedguard oil filter which traps a little less dirt than the toughguard. I decided to run it anyway and see what the mileage says, as I was running 10w30 and the mobile 1 website says there is just an extra additive in the truck/suv oil. And i have to pull a trailer of shingles with it so i might like to have a little more protection. I did exceed my liked change interval of 15,000 by 6,000 but I couldn't get the oil and drove it light.

It's M1. You're fine. There won't be a discernable difference in FE between the two. But 21k miles on oil? I salute you, brave one! I don't go longer than about 9k miles on full synthetic in my car since it's getting very nasty after ~8500 miles.

If you haven't changed them yet, I'd look into changing your lower intake manifold gaskets. Those can leak coolant into your cylinders when they go bad.

I use 0w30. It's has better ratings than 5w30 for protection, but it's a lower cold viscosity. And, M1 has better mileage claims on the bottle, so it's probably true. I put the stuff in when I put the van on the road, but I was getting better than EPA right the first tank I put the van on the road, before I really knew how to drive it.