Too High Gas mileage after oil change

[gkaufman@tiac.net (gkaufman)]

As some of you may remember, my oil temp on my 86 Coupe GT had been running 
high.  I just had an oil change and instead of using all 15W40 used 3 qts of 
20W50 and 1 quart of 15w40 (Penzoil).  I change the oil every 3000-4000 
miles and currently have 75K miles on the car.  

My gas mileage has been phenominal over the last 150 miles, ranging from 
30-35 miles per gallon.  Normally, I get 23-27 mpg at comparable speeds.

I am suspicious of a problem here.  Does anyone have an idea of what could 
have happened? Thanks all.  glenn