See any MPH or ET improvement from changing oil brand or weight? If so, from what to what.

I doubt you'd see much change in MOST cars, maybe a thou or so. Maily because the lighter the weight the less the resistance and syntho oil is slipperier

I have seen 10 hp differences on a DTS engine in using 30 WT oil at 130 F oil temps versus warmng the oil up to 170F. I started using 5w30 and 5W20 wt. oil and now see no differences in HP gains at 100F and 190F oil temps I have been told by NHRA stock class racers that 5W20 synthetics are worth a .1 to .18 in ET depending on the old oil that they used before switching I do know some Comp Eliminator racers that use 0 Wt synthetic oils that change the oil after every round because the oil is worth .003 ET gain and the motor will blow up if you run it twice at WOT, these are high RPM N/A motors that make 2.25 and up HP per C.I. , That is insane to me but I don't use it.

As mentioned, depending on several variables, the benefits can be from minimal to obvious! With the right synthetics, engine wear can be reduced and operating temps can be lowered also in addition to freeing up some HP. All synthetics are not created equal either. Personally, I've used Amsoil for years in my personal, tow and race vehicles and have seen all of those benefits. Oil, trans fluid, gear lube and wheel grease.

Amsoil source and info site.