The scheduled oil change is every 20K. Do you think I should do interval oil and filter change or just a filter change in between the scheduled oil changes? Thanks

 

I don't know where you get the 20k from, but, in any case, it makes no sence to change the oil and not the filter. Doing so mixes dirty oil with clean oil, giving you dirty oil.

 

I have seen some tests that indicated the oil outlasted the filter. So, just changing the filter is a possibility. To know for certain, you would have to send your oil out for analysis. There are a couple places people have recommended. I haven't done this yet. I've got less than 4,000 miles on the current oil.

http://www.dysonanalysis.com/premiumkits.html

http://www.blackstone-labs.com/standard_analysis.html

 

I've used synthetic oil in the past where it was recommended to change the filter between oil changes. 15000 mi. oil change, filter at 7500 mi.