White smoke from exhaust

Bought a certified Toyota Avalon XL 2001 with 58K miles on it. Car runs and looks like new. Only worried about the white smoke that comes out of exhaust every time I start it. Did not see this when I bought it two weeks back. Been on a long trip after I bought it and started noticing the smoke. It stays for a couple of mins after I start the engine. Coming back from my long trip, checked with the dealer and they said the engine oil was less as they did not fill it properly thinkin its a V4 engine. They changed the oil right this time. But the white smoke is still there. It comes not only during cold start, but also during lunch time or in the evening. I am worried as its my first car and do not have much idea about cars. Please advice.