I use Rotella T 5/40 synthetic and I do not see any oil loss until about 2500 miles into a change and then I am only adding 1/8 of a quart when it is low. My engine has 146,000 miles on it.
Even though your consumtion is higher, I would not worry too much about it. You could get an oil analysis to make sure everything is happy though. I would also make sure you are reading the dipstick under the same conditions each time. Our dipstick is a bit dodgy to read.