Lower MPG right after an oil change?
 

I've got what appears to be one of the good '04 Auto RX-8's; hardly any problems, pretty decent gas mileage, and fun to drive! As far as oil consumption, (mix of country and city driving), I've found that I need to add a quart of oil every 2K miles. I've been following an alternating pattern of oil change, then add a quart 2K miles later, then oil change 2K miles later, etc. However, I've found that my mileage drops slightly (just above or below 20 mpg) after an oil change. Then, as I get more mileage past the oil change, my mileage increases, so I'm trying the following pattern: an oil change, then add a quart after 2K miles, then two more adds before an oil change (8K between oil changes). At this point, I'm 5K+ after last oil change, and the mpg on my last tank of gas was 23+.
Has anyone else experienced something similar? I obviously want to increase my MPG, but don't think I want to go more than 8K between oil changes. Any thoughts?