My 02 Jeep 4.0 160,000 miles jeep recommends 10w30 in the summer and 5w30 in the winter why? Temp here can hit a low of -40 but average -30 for about 1-2 weeks. Summer 70-85 (seen 100 maybe 15 times in 30 years. Or we can use Arizona in the desert. If 5w flows faster at start up weather it's hot or cold outside and both oils are 30 weight at operating temperatures what advantage does 10w30 have over 5w30?. I tried an experiment winter 2016. Left 5w30 Valvoline HMSB in during the summer and found 5w30 to be quieter and (piston slap) went away faster at startup summer and winter then 10w30. The slap is gone at 210 temp quicker with 5w30. Is there some advantage to 10w30 that I am not getting? Is it all about oil shear rate between the two oils?. Are oils so much better today then 16 years ago. Would jeep change their recommendation if the oils today we're around in 2002? Does conventional shear more then synthetic or synthetic blend Thank you.