Hi...If it smokes on start up...It's worn rings, but if it starts to smoke after running for a few minutes then it's a blown head gasket causing pressure to build up in the crankcase, and forcing it through the oil breather tube into the carb. Check to see if the carb has any oily residue in it. Also what grde of oil are you using. Should use a sae 30 weight, not any of the lighter multigrade oils. Since it's a 12hp 38in tractor it must be close to 20 years old now and another thing to check the dipstick if the oil smells like fuel. These mowers have a tendency to have the fuel float getting stuck open, and feul going through the carb. into the oil breather tube into the crankcase...leading to thinning oil, smoking, and leaking....good luck