2007 wolverine 350 smoking

[allthrottle92]

recently let a friend ride this bike, was 51 and would not give it gas and go. think it might have got too hot, because the valves were ticking bad. cooled down and was fine.
Changed the oil and it started smoking when it starts up. kind of a greyish white smoke. little burnt oil smell. does not continue. once warmed up the bike is fine.
Anybody got any ideas why it is doing this?

[366warrior]

usually when a motor smokes upon startup then goes away, the valve stem seals are shot. sucks because they cost only a couple bucks but the top end needs to be taken apart to replace them. not sure how overheating would destroy them although they are made of pliable rubbery material.