I just change it anytime I feel like it under or up to the recommended change interval of course. The manufactures recommending change interval is exactly that a "recommendation". If I feel I have driven the car hard over a certain time or if its been a bad summer heat wise I change it. If its been a snowy winter with not much driving I wait a little.
I use semi-synthetic and change at 4000miles give or take about a 1000.

yal
88 230E (W124)