Almost certainly oil due to wet sumping during lay ups pushed up by crankcase pressure into inlet. Once burnt off should be fine and a reminder to use vehicle regularly.
If that's the case, why only smoke on one side? The crankcase vent tube is dead center in a siamesed intake manifold so any oil introduced would feed both cylinders.
An exhaust valve guide seal on the left is much more likely.
To confirm: after a longish layup unfasten the left side exhaust header at the head and examine the exhaust valve. If it's wet, then you have a guide seal issue.
