Synthetic oil is the tipoff. Modern engines do well with synthetic oils in automotive applications such as cars, trucks, tractors, and the like. BUT....every small engine manual I have looked at recommends AGAINST using synthetics stating that excessive oil consumption and smoking will generally be the result.
Synthetic oils generally have a viscosity rating of 5 weight or less, and thin out less in operation and when the temp rises. Conventional oils generally have a higher viscosity rating in the 10 to 15 weight range. While the engine may benefit from higher film strength and less breakdown under high temp operation, it will also allow more to slip by the rings and valve guides - causing smoking.