When my dads 2002 Suburban 5.3L with 124k miles was in for a u joint repair, the tech said he noticed the rear main seal was leaking. Now, I've never seen a single spot of oil on the driveway, so I assume it only leaks when the engine is running and the pressure pushes it out. It needs a quart of oil every 1300 miles or so, but I always just assumed it burned it. It's been losing a qt/1300 miles for over 5 years now. Anyway, the point of this is to find out if moving to a different oil viscosity will help it out. I have been putting PYB 5w-30 in it for years now.