02 Jeep 4.0, 165K miles. 50-50 hwy/s&g driving, hot in SoCal. Don't want to spend on expensive oils at this point, change every 5-7K miles. Prev. owner used Econolube places, randomly I think. So going with ST, HM 10W-30, -40, conventional or syn HM 5W-30? Advice?