jeep recommends 10-30 all year, I think. It should tell you in the manual if you have one. If nothing else there is always trial and error. I would run 10-30 and switch to 5-30 with any problems. My motor started blow by so bad that I ran 10-30 then 20-50 then mack bulldog15-40 diesal blend then finally straight 50 before I tore the motor down. It was still running beautifully with the exception of it lokking like a fire extinguishers was going off constantly under the jeep going down the road. I drove about 40miles a day and it burned 4qts a day. Good thing I worled in a diesal shop at the time with bulk oil tanks that nobody monitored!! I carried three, gallon washer bottles full of oil behing my rear seat all the time.
