I use Chevron Delo 15W-40 all year in Southern California. Temperatures here range from the upper teens here in the mountains right now to over 100 down in the desert in summer. 15W-40 is heavy duty "fleet" oil sold in gallons for diesel and gas trucks. It has more detergent/dispersant and ZDDP anti-wear additive than passenger car oil. Pennzoil LongLife, Shell Rotella T and Mobil Delvac are other popular fleet oils.

Jim

Jim