Creeping oil temp over the years

I fly a 912s since 500hrs, the installation has not been changed and the engine now has 700hrs. I have the impression that the oil temp has been risenby some 3-4 deg celsius over the last year, maybe two years (100h/year). The result is an oil temp between 105 and 110 deg celsius (the beginning of the yellow arc) when flying 75% at 2000ft when the OAT is higher than eg 27 deg C, which has not been the case before.

The oil cooler is located in the airstream behind the water cooler, but CHT appears to be unchanged. Assuming that nothing blocks the airstream and oil level is normal, do you have any idea of probable causes? 

Is there a mechanism in oil coolers similar to vintage cars where soot may partially block the water radiator? Any ideas?

Kind regards 

Alexander