Run a 10w40 HM in it, I am partial to QS All Mileage, Maxlife, and Castrol GTX HM, that's what I run in my '93 5.7l C1500. Or as others have suggested LM Motor Oil Saver and a 40grade of your choice. I have never had any M1 HM oils resolve a leak myself, so I would not use them personally. Never used ST long enough to weight in on them with seals but I would try them.