I have been using Valvoline Maxlife in my '94 GMC 1/2 ton with the 5.7 for several years now with no complaints. I decided to give Castrol HM a try. With Maxlife, I never needed to add oil until my next service was due but my oil pressure would drop off after 1500 miles. With Castrol HM, my oil pressure is a lot better but I am having to add a qt after only 1000-1200 miles. Any ideas?