15w? Are you having low oil pressure on start up as well as hot idle? I can see 10w40 because I already use 5w40. What do you think the benefit of going to 15w40?
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No low oil pressure problems at all, ever. I wanna say the lowest pressure I've ever seen on my dash gauge was 25-30psi. This is on a 200,000-mile 5.3. I recently completed a teardown for internal inspection and AFM delete along with ported stock-replacement oil pump (with new O-ring), plugged AFM towers and plugged relief port in the oil pan. My hot idle pressure now is ~33ish. Cruising at 50MPH/1,450RPM, the gauge reads ~43-44ish. At 5,000-6,000RPM, it'll hit around 70psi.
The benefit of going to a heavier oil is less shearing of the oil barrier. You need a lighter oil in winter because, as fluids do, they get thicker in cold/freezing temps and won't flow as easily. This is where the 5W, 10W, 15W, etc. come into play. The second number is for the thickness of the oil at operating temp. Car/engine manufacturers also factor in efficiency losses from heavier oils. A lighter oil moves more freely, reducing pumping and movement resistance, so the engine is a fraction less "bogged down" by the oil. This equates to better fuel economy, but it's so minimal that you'll likely never even realize it. The manufacturer has to explore all possible avenues to squeeze out every last bit of efficiency to satisfy EPA requirements. The trade-off is slightly reduced engine life. Modern engines are able to be designed with such precision and tight tolerances that they actually need lighter oil to be able to have that barrier between the parts (journals and bearings, etc.). But, after a good, long 100,000-mile break-in, these tolerances are opened up a little and the lighter oil can't make such a strong barrier. It still protects better than metal-on-metal, but it can be "squashed" thin at higher engine loads and temps. A heavier oil can create that stronger barrier/cushion in the now wider tolerances. I think 15W-40 is fine for my climate and should be for 99% of the year.
