What kind of OCI would you recommend for a 240 (n/a B23F) that is driven about 5-6K miles a year? I have been changing it once a year and not paying much attention to the mileage. We live in Northern California but not far enough north that it snows in the winter. Temperature is usually between 60-70F year round, with a few weeks of 40's in the winter and a few weeks where it climbs into the high 80s/low 90s in the summer.

I usually run Rotella 15w40 and a Mann filter.

I'm asking because I figured this was a "safe" change interval given that the car is not driven much these days. But recently I have noticed that the oil is dark black only 500 or so miles after changing it. I don't recall that being the case in the past. Time for a Seafoam treatment perhaps?