I was getting about 31/38 city/hwy with 93 oct. gas in my '93 Corolla 1.8L using Maxlife Synthetic 10W30. Pretty good, I think, at almost 150k.
When I switched to plain old basic Valvoline 10W30 for an Auto Rx cleaning cycle, it dropped by about 4 mpg. Seems huge, but I've checked and rechecked the math, and it's true. Warm up time is significantly longer right now, which must affect it, although winter in GA is mild, and I've not been running the heater, which makes a huge difference in engine temperature, on the assumption (guess) that higher temperature makes the Auto Rx more effective. I wonder if the Auto Rx itself affects it somewhat?
In about 100 miles I'll switch to Castrol GTX for the rinse phase, and carefully document mpg.
After that, back to synthetics and hopefully mid/upper 30s mpg again. Hoping the Auto Rx also conditioned the seals well (no consumption or loss at all in the cleaning phase), I just placed an order for Redline 10w30. We'll see how that does after 2000 miles of GTX. The weather will be hot again by then, so that will presumably affect things, too. That, more than anything else, seems to affect mpg on this car.