So I was ripping the front end off my 99 last weekend and to my surprise I found an oil cooler. So I got to looking at my C3 and voila,oil cooler. My 99 has a 5.3,the c3 is an 01 with a 6.0. Just thought I'd throw that out there. These trucks came standard with an oil cooler,which makes me reconsider using a 30 or 40 grade. I think they'd be fine with a 20 grade if oil temps remained low,but that's my humble opinion.