FWIW the last two lubricant classes that I attended (courtesy of Uncle Sam) Shell Rotella (diesel dino, not synth) was mentioned for older ICE (internal combustion engines) due to it's higher levels of zddp, a zinc additive that is deleted in motor oils for gas engines because of cat converter contamination issues. The zinc helps with the top end, at least that is the theory. As for multi-vis synthetics, you may experience better wear resistance (less tendency for shear) with a slightly higher viscosity (10W or 20W instead of 5W). That 5W-40 is great in winter for making it easier to spin, but I'd be afraid of low oil pressures on really hot days in a worn engine. My jeeps are high time 4.0's so I use the synthetics (5W or 10W-30) in the winter, but usually shift to good multi vis (20W-50) dino oil in the summers. I average monthly oil changes & use a good filter. I'm able to maintain good pressure all year round that way, and the synth tends to keep the engine guts clean.