Another kicker to synthetic oils is putting them into engines with seals that have lived there life with dino oil, they tend to leak oil after the switch. The synthetics are slicker than dino and will slide past older seals. Went through this one before with my COMANCHE. It didn't leak anywhere, I switch to CASTROL SYNTEC for extended service intervals and it looked like an oil shower afterwards. Switched back and the leaks stopped shortly there after.

i run rotella in every thing I own

15/40 is a bit thick for alot of applications. My 4.0 has a hard time turning over on a cool morning with 10/30 in it. To bad they dropped the 10/30 ROTELLA, that was some good stuff.

still find the 10-30 Rotella around here, and whos kiding who, you live in Fl, it never gets cool down there. haven't had any problems start on cool mornings with my Cherokee( gets just below freezing during the winter, but not for long). I'll tell you what I do have a problem with, when I start the Cherokee, it doesn't start right off like a fuel injected motor should, it has to spin over a few times before it lights off, I haven't even checked anything yet, any ideas?

Fuel pump? Slow starter from too thick of oil?(couldn't resist) The 91+ SBEC and JTEC should be able to start in about 2 revolutions of the crankshaft. My 88 takes a little cranking to get going due to the magnetic pick-up and sync set-up.

yep been there too with my old wrangler i had with the 4.0. the reason why syn oils tend to leak rather than dino oils is due taht syn oils molecules are pretty much uniform in size and that size just happens to be smaller than dino oil. where as dino oil has larger molecules with some smaller molecules but with larger unrefined ones in the mix therefore using the theory of blind probability there are more larger molecules at a given point than smaller ones with the dino oil. here is the kicker there is a difference between regular syn mobil one and high mileage full syn mobil. that difference is seal swellers. check out mobil's website they admit putting them in the high mileage, because that is how to keep the small molecules inside the crankcase with out putting new seals in. and not all the time will re sealing work becuase of the metal rotating part being worn from the old seal sort of a devil's advocate of friction and erosion. in other words if you want to run syn on an older motor get the high mileage stuff.