Strictly from the filtering standpoint, changing the filter at 4-6K is most often a waste. Lots of variables and potential caveats to that:
- Is one of those midget filters with a small amount of media? If so, and you don't know how much the filter is loaded over the interval, it would make more sense to change.
- On the other hand, most engine don't generate much contamination and seldom, if ever, is the media loaded to anywhere near capacity in
- Filters actually filter better, become more efficient, as they load up, so a long interval actually yields better filtration towards the end of the interval.
- Running a double interval exposes the filter to more chances of failure... seals, ADBV, bypass valve, etc. The likelihood is small with a good quality filter, but it exists. Have not seen any test data on this but, anecdotally, the likelihood of a failure in a double run seems low... IMO, a single digit, or low double digit risk percentage.
- You are leaving a certain amount of old oil in the engine but the new oil will pretty much rejuvenate what's there. You definitely wouldn't want to remove, drain and replace the filter. It can, and is, done (done it myself) but it adds some risk of seal failure. At a 4-6 K interval, for the most part, the oil is not fully "used up" so I personally would "feel" OK to leave that small amount in. I would hesitate more if the oil was really used up or contaminated from fuel dilution.
Overall, in my opinion, doubling the interval on the filter would be viable if:
a) You weren't a short tripper, lots of short hop city driving where the engine doesn't get fully warmed up.
b) You run very long OCIs (you don't).
c) You have a filter with a tiny amount of media.
All that said, unless you are a short hopper, I suggest you run a longer oil change interval (7.5-8K) and THEN change both the oil and filter. Unless your driving situation is "severe" in some way, at 4-6K, you are most likely draining viable oil and tossing a viable filter. If saving money is the root of your question, then running out to 7.5K, at least, answers that from both the oil and filter standpoints.