Changed the oil last night in the 4.0L w/76k. I had been using Trop-artic syn-blend 10w30. But I recently bought some Pennzoil HM formula for my magnum V8 w/178k. However, I did not ave the heart to switch the V8 off of the Mobil-1 HM so I had to find a use for this Pennzoil HM dino. I put it in the 4.0L and I immediately noticed it was quieter & smoother running. Heck I almost wish I'd have given it a try in the 5.9L but I think the main reason the Pennzoil HM worked so well in the 4.0 is because of the combination of the flat-tappet motor & a robust additive pkg...not sure if it would hav mattered as much in the roller magnum motor? Just thought I'd share and give 2 thumbs up to Pennzoil HM formula.