I've read where some member like diesel oil in their GN. I'm curious why. Is it the 15/40 weight? I was in Oreilys today and came close to buying some Royal Purple Duralec 15/40 Synthetic. I currently run Mobil1 10/30 with ZDDP. Would I gain/lose anything with the RP? Car has 75,000 original mile, stock lower, zero issues. Oil PSI, 15 hot at idle.