vehicle and engine : Jeep with 4.0L six oil choices: maxlife 20w-50 or castrol/delvac 15w-40 reasons for other then 10w-30... slight rear main seal leak and low pressure once engine is hot and at idle. I used 20w-50 last summer and had good pressure. I will be using auto-rx with the last 750 miles of current oil change. I will also be using a motorcraft FL-1A filter. Other concern besides pressure is the leak.. would a thicker oil be less likely to leak? Also cold start issue.... temp rarely hits the teens, most of the time lows in 30's and highs between 30-50 in winter and in summer its like 70's for lows and 85+ for highs in addition to high humidity... thoughts please.