increase in mpg (bit of a surprise)

Forgot to mention - it's got just over 135k miles on it, and it's a

I *did* change to synthetic ATF in the diff, but that was a few months back, and before my last 3 readings of 28, 29 and 30 mpg...

Is this about the time of year when gasoline blend changes from "winter" to "summer" ? IIRC that can account for a change in MPG on the order of magnitude that you observed.

Warmer weather.

If the auto-rx has been in there for 1,000 miles it is time to drain it out and change the oil and filter. If you're trying to track 1-2 mpg changes from one tankful to the next, you're seeking an accuracy that is not generally possible. Most likely variance is that the auto shutoff on the gas nozzle doesn't always switch off at exactly the same point and you need to watch mileage over a number of tankfuls to get an average reading. When I bought my 1977 Toyota pickup truck, the gas gauge only worked for the top quarter tank - from full to 3/4 full, so I always very, very carefully watched the number of miles on the odometer and filled up again at least 50 miles early. One day the truck died on the freeway and I had to coast across fast moving traffic to get to the offramp and to a mechanic. Took about an hour before he finally correctly diagnosed the "no-start" problem to "out of gas." (even though the mileage hadn't been reached) Moral of this story is that there is variance in pump fill ups.

Directions said to change 1500 miles after...

Besides, isn't it supposed to be the oil *after* the autorx treatment that has to be changed after 1000 miles?

They must have changed. Used to be change after 500 miles.