Along with my '89 K5 I have a '99 4X4 Suburban with a 5.7 Vortec engine. I'm using Mobile 1 15w/50 in my Blazer and Mobil 1 10w/30 in my Suburban. Since the cost of oil is almost $5 a quart I have been thinking of switching back to conventional oil and was wondering if anyone is using anything other than 10w/30 in a Vortec engine. The owners manual suggests not using anything above a 10w/30 but here in Texas I want something that won't thin out. I use 15w/40 at my Mercedes shop and was considering using it in my Suburban. Any thoughts?