My TJ is getting a bit tired so when I change the oil I throw in a couple table spoons of Geritol and it perks right up.
I also prefer conventional motor oils in my TJ so long as I don't move to the frozen north where synthetics flow better in extreme conditions. I would not run a 10W-40 though. The factory recommends nothing heavier than 10W-30 and 10W-40 is just harder to pump and get into the nooks and crannies. The old days of running a heavier viscosity oil as engines age is also outdated, that was done before today's engine tolerances got so tight as they are now. I sure wouldn't run anything heavier than 10W-30 in any of my high-mileage cars or the Jeep.pennzoil 10w40 dino. tried running mobil 1 synthetic once and got RMS leak within days after the change when I never had a leak before. put in the dino 10W40 and not a drop of oil leaked since
I also prefer conventional motor oils in my TJ so long as I don't move to the frozen north where synthetics flow better in extreme conditions. I would not run a 10W-40 though. The factory recommends nothing heavier than 10W-30 and 10W-40 is just harder to pump and get into the nooks and crannies. The old days of running a heavier viscosity oil as engines age is also outdated, that was done before today's engine tolerances got so tight as they are now. I sure wouldn't run anything heavier than 10W-30 in any of my high-mileage cars or the Jeep.