Why is there a bit of smoke coming out the tailpipe of my 2005 camry with 90,000 miles

This is a 4cyl engine with 90,000. So first off it will sit for 5-6 days. Then I'll use it. On start up everything is as usual, I always let it idle till the rpms come down, then I go. I'm on the freeway, 60mph. I floor it, it will kick into what I guess is called 'passing' gear, I see gray smoke coming out the exhaust in my rear view mirror, I slow down to 60 and do it again, gray smoke, but less, the 3rd time maybe a little bit of smoke maybe not.
What do you think this is. I take it in every 5000 mile to have an oil, filter, fluid etc. change. I'm usually about a quart or less low.