Model boat w gas motor/nitro
'Is it Safe' is an unanswerable question. All action, including inaction, carries some sort of risk - much of it unquantifiable, since we cannot predict the future.
You may be asking 'How much will a lake be damaged by running an I/C motor?' - that also is unquantifiable. More likely you are asking 'Will I be allowed to run an I/C motor on a lake?'. The answer to that will be in the various local laws covering that amenity.
If we are just considering oil pollution, technically, all motor craft may leak some oil from the prop-shaft if it is lubricated. Exhaust gases may carry some oil from an I/C engine overboard, and will emit quite a lot if they use a total loss lubrication system (mixing oil in with the fuel). Pretty much all 2-stroke engines use this lubrication method. Steam engines also mix oil with the steam...
You can add an oil trap in the exhaust pipe to capture much of the emitted oil. That's generally a good idea, because it's messy. I was doing this for just that reason in the 1960s when environmentalism was a crank religion...
The actual danger, if any, depends not so much on the fact that some oil may be emitted, but on the total amount. But proportionality is no longer of any concern in today's world of manufactured panics.
