Castrol synthetic or Valvoline?

FWIW... I have been using Mobil 1 since it came out in the 70's. Raced on it a bunch (in rallying, where you are pounding on the engine 100 miles at a time) and found that the oil breakdown (oxidation) is far, far less than for the best brands of regular petroleum oils. The reduction in wear in some parts is phenomenal from my teardowns; cylinders and ring and bore wear down is reduced 80-90% in my checks, and bearing wear is waaaay down also. Wear on lifters and cam do not seem to be changed, and the incidence of cold start lifter rattle seems more with Mobil 1; this IMHO is probably due to the lower cold temp viscosity. Protection of the bearings after oil loss is crazy better versus standard petroleum oils; it has saved my engines a couple of times.

As far as the viscosity versus temp: Viscosity differences are checked at only 2 temps: 100C and 40C , so neither reflects the cold temp behavior well. True synthetics will maintain a lot lower viscosity at temps from 30F to -20 F or less. So if you run a heavier synthetic weight like 5W-40 and the temps get in the teens in AZ in winter, your oil will flow well at cold start-up. You can see the improvements in low temp viscosity reflected in a spec called "Pour Point"; true synthetics all run Pour Point temps 15-20 degrees F lower than their straight petroleum counterparts.

A lot of synthetics are blends as has been stated. I can't say much about them. BTW, Mobil 1 is actually petroleum oil that has been highly, highly refined. I understand that the objective in the refining process is to get the oil molecules down to a very specific range of sizes. Somehow, that gives the performance benefits.

Yes, I recall the issues reported with oil leakage in the early days of Mobil 1. It tended to occur in engines that had higher miles and worn seals. If you switched to it at say 5k-30k miles, there were no seal leak issues.

Check the zinc additive levels in the the synthetic weight that you select. The lighter weights tend to have lower zinc levels; I think this is more EPA mandated for these light weights, and is perhaps a downside to the lighter weights. My son is going to change to a 10W50 Mobil 1 or thereabouts to get the higher zinc numbers.