Last things first, you should change your oil somewhere between 4000 and 5000 mi if you use synthetic oil IMHO. Also IMHO you're nuts to use dino oil in an RX since they run so hot normally. At the rate you mention of 400 mi per month you'll end up changing your oil about once a year, $23, $25, even $30 is just not that much money to spend once a year to know you're engine is being protected properly. Also, you should invest in a top-notch oil filter as well since it may be sitting on there for a year, spend the $10 and get the Mobil1 filter.
Fwiw, I make the same assessment and only use synthetic at about 5k, a few bucks a year to protect an engine worth thousands is well worth it to me but i leave the filter selection up to the dealer or local fast oilchange place that has Mobil1.