All else being equal, the CHT and oil temperature will rise and fall with ambient temperature as the rate of heat transfer is proportional to the difference from ambient.
So you would expect a seasonal variation as the ambient temperatures change.
If that doesn't explain it then there could be one or more other explanations e.g. gauge error, baffling leaks, induction air leaks, magneto timing, valve sticking/leaks .....
Are there any other changes such as poor starting or idling?