My Piper Archer running a Lycoming O-360-A4M suffered the same issue which had nothing to do with the type of flying. I fly both IMC and VMC. As already mentioned it's just down to the ingestion of moisture on the ground and may well be a sign of underutilisation.
I found the remedy was to take the aircraft up to 10,000' in a continuous climb once a month and then do a powered descent over an hour's flight time. I get my oil analysed on every check and the residual water content dropped dramatically after I started doing that. Check your POH but I start leaning back just a little after 5000' with a little reduction from full power to be kind to the engine.
Keep an eye on your oil temperature throughout. The hotter the better but of course staying in the green.