First car I ever bought nearly 21 years ago with 7.3 miles on odometer today has just over 414k trouble free miles on her never missing oil change intervals of 3k or less using 10w30 winter 10w40 summers conventional of course. SO NOW while she still runs like a swiss watch I'm caught up in reading how people with high mileage 4.0's use 15w40 weights and even switched over to synthetics!?! The wife prays it blows itself up because I keep the 70k dollar suv outside in bad weather over my beloved first ever car, I Tully understand and relate to that movie "Christine" I just wanna do what's best for the engine. Zero oil burn and only thing I notice is she runs hotter than she used too back in the day, maybe a heavier oil might remedy?