Lou, I don't remember exactly how much I put in when i did mine ( 2005 envoy) this summer but I used CAT extended pre mix. I think it was about 13 gal. I bought more than I needed, took some back to CAT and kept two gal for topping off if needed. So I would recommend buying 15 gal, and then you can take back what you didn't use. I replaced the thermostats at the same time which I also got from CAT. good luck Jim
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