Engine toro wheelhorse riding lawn tractor 5 years old

[cheny]

for the last 2 summers I have started my lawn tractor after 10 seconds I have white smoke coming out the exhaust for about 5 minutes then goes away the oil level has always been full I was thinking it's the gas put in higher octane gas I was told it could be the breather where is that located, if it was a head gasket problem won't it smoke all the time thanks for any help