I have some Mobil 1 Tri-Syn 0w-30 in my car for the last 1K miles, but I'm wondering if I should dump it and use Pennzoil PureBase 10w-30 for the Summer months here in hot central Texas. The 0w-30 Mobil 1 TS was bought on the cheap ($1.30/qt on clearance) and it was great in Winter, but I'm worried its HTHS is not going to protect well enough for Summer. My engine isn't a turbo, but does have a 7K redline I get to fairly often. And I know the Pennzoil 10w-30 PB has a good HTHS rating. If I drain the M1 and put in Pennzoil 10w-30, it'll just be for a few months, then I'll go back to M1 0w-30 TS, of which I still have 8 qts. What would you do? P.S. I know, I know - it was probably a mistake to get the Mobil 1 0w-30 in Texas.