I have a 95 4.0 in my Jeep. I would not use a 15w-40 Delo in mine, because my pressures would be to high. With Castrol 10w-30 (one of the thicker 10w-30's) my oil pressure was pegged (80 psi)while cold (70f). With Pennzoil 5w-30 my pressures are great (60 psi cold). I don't think this oil will be good for your catalytic converter.
I'm running Shell Rotella T Synthetic 5w40 in my '88 Cherokee with the 4.0l. Cold start pressure is the same as when using 5w30 or 10w30 - sits at 60 psi.
Supposedly the Delo is availible in 5w40 or 0w30. Never seen it, but I'm sure a distributor could get it.
Two schools of thought:
One, running 10w30 these motors run forever regardless of oil brand so long as service is regular. Use any old 10w30.
Two, there may be some evidence that wear numbers may be lower on an xw40 oil on these "old" motors. If that means the difference between 200,000 miles and 250,000 miles out of the motor without rebuild, well .
My '88 owners manual suggests that 20w50 oil is OK above 32 degrees F. 10w30 and 10w40 above 0 Degrees F. 5w30 for temps below zero.
I'm guessing 5w40 was not a real common grade in '88!
[ October 17, 2003, 01:17 PM: Message edited by: MNgopher ]
My 93 GCL with that engine was very happy with Delo. Normal pressures between 50 and 60, close to 80 when I found myself in the mines at 14,000 ft above sea level and -10C. I might not put it in when the engine was new, but I got it at 55,000 miles and sold it around 80,000 3 1/2 years later.