I've seen a lot of stuff about upping the viscosity in an engine with higher milege. I'd think it would be even more applicable with previously lackluster oil changes. I just bought a 94 Corolla with 70,000 on the odometer. Would a 15w-40 be a better choice for the summer months? Where I live, it regularly gets to over 90 F, and I travel to areas where ot goes over 100F practically every day in the summer (Pueblo Colorado). My manual recomend 10w-30 year round, but doesn't specifically advice against using a heavier weight oil. So would the milege on my vehicle warrent a 15w-40 for summer use? I don't wanna run 10w-40 for shearing reasons. Thanks