[Huskymaniac]

I'm glad people are finding this informative. It has actually pushed me toward increasing my OCI. I am thinking of pushing it to 6000 miles and here is the reasoning.

Subaru suggests an OCI of 7500 miles or 3750 miles if one drives in harsh conditions. Harsh conditions are extreme cold or heat or driving in a dusty environment or making lots of short trips. The only thing that applies to me is the extreme cold or heat and that is only for about half the year. Splitting the difference gets me 5625 miles.

If I am thinking about longer OCIs, I will only do it if I use a synthetic like Mobil 1 which claims to be good out to 7500 miles regardless of conditions. So, really, my oil should be in good shape beyond 5625 miles if I am using a synthetic.

While the oil may hold up in terms of not becoming excessively oxidized and acidic, there is still the issue of contaminants finding their way into the combustion chamber and, hence, the oil and oil filter. That is why people still change oil filters when they switch to synthetics and extend their OCIs. But the larger Purolator and K&N filters are 60% larger than the stock filter. 60% more miles would equate to 6000 miles if the reference point chosen is the lower 3750 mile number.

So, given that I don't drive in extreme conditions year round, I am switching to Mobil 1 and I am using a 60% larger filter, I think a 6000 mile OCI is fine.