My 89 Cherokee with the straight six (same motor as the wrangler if I remember correctly) had 260K miles when sold and was on a diet of 10w30 Castrol GTX. In all honesty I think that 4.0L six would go forever on just about any oil. I know of one used as an irrigation motor and gets straight 30W bulk oil once a year, 24/7 for six months, and it keeps going and going.