Ok, that's a fair point but then my question is, why would one want to mix old oil with new? Even though it's maybe only half a liter but still? I'm not judging, just curious what the reason is behind changing oil more frequently than the filter?
Here are a few reasons why I might prefer to skip a filter change for every other service.
1) Time. Less time to complete the job if I don’t have to remove the filter.
2) I couldn’t find my filter removal tool yesterday so I was looking for an excuse to not have to change the filter. This may be bone idle laziness on my behalf.
3) In this case I have only ridden about 500 miles and the oil looked like new anyway. But since it had been 18 months I thought getting the old oil out made sense, even if a small portion got left behind.
4). If it truly isn’t necessary, it is a waste of time, money, effort and it is one more petroleum laden oil filter in a landfill somewhere.
5). I could get three bikes done in the time it takes to do two if I don’t change filters. If I only had one bike it wouldn’t be such a big deal.
I appreciate the back and forth so don’t take any of my reasons as argumentative.