It could be the type of usage and the duration of the runs are too short for the engine and the oil inside to reach normal working temperature. Lots of cold starts and short runs will inevitably cause some fuel contamination of the engine oil due to the relatively large quantity of fuel used during a cold start, much of which is not burned and some of which will find its way into the oil.
An hour of normal use would usually be enough to evaporate most of the surplus fuel from a small number of cold starts but some fuel residue will always remain, often leaving behind a fuel odour for those with a sensitive nose. This would be exaggerated if the oil pan capacity was quite small but the residue will do little harm over the short term in a quality oil.
It is possible there is a fuel problem, such as a small amount of fuel leaking past the carburettor float valve, which if there is no fuel tap and the system is gravity feed, some could find its way into the engine intake.
In this case cold starting would be possible without engaging the cold start enrichment and it might be noticed the oil level never seems to reduce or even begins to rise and fuel consumption would be noticed to be higher than normal.
If the engine is being used in a cold climate it is possible it is over-cooled and even when worked hard for long periods the oil doesn't reach a high enough temperature for it to do its job properly.
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