What is your opinion on high mileage oils? I have a '72 chevy c10 with a 350 and have been running Rotella with some mmo and a wix filter. I am about to change it out for 3 quarts of pennzoil high mileage 10w-40 along with 2 quarts of castrol hd-30 for a little more thickness on start up. it currently oil fouls plugs but it now just has a small miss since I started using tcw3 in the gas along with changing out the 5w-30 royal purple i had (brain [censored] when I bought it). I also have another gallon of syn 5w-40 rotella along with 15w-40 rotella, 3 bottles of stp 4-cylinder treatment, and a full bottle of mmo if I should use any of those things aswell or instead. I know I should order engine rx too and will soon. I also got a compression tester to find if it's the rings or valve seals that are causing the oil fouling but am away from my truck right now. Do high mile oils really do anything for cleaning up the ring packs and improving seals? I know not to expect a miracle in a bottle with this high mile motor. And I couldn't find anything on the search.