Gas or Condensation

I have a 2001 Ford Expedition XLT, I believe it is a 6 cyl. The truck has 115,490 miles on it. When I got the truck, I was informed that I could use just regular unleaded gas. But, I normally use the mid-grade unleaded gas in my truck, but lately I have been using the regular unleaded because of the gas prices. Sometimes when I crank my truck, a white cloud of smoke comes from the exhaust. This does not happen often, but enough to be embarrassed. I took the truck to a mechanic and he told me that due to the miles, the smoke could come from the condensation. With me being a female, I am not familiar with know anything about condensation from a vehicle. Would you have any suggestions on what may be the cause of the smoke from my truck, the regular unleaded gas, condensation or maybe something else? Please HELP!