I use Royal Purple synthetic:
www.royalpurple.com/
It's more expensive going in but requires less frequency. I also purchased a very good one owner RX (2001). The previous owners were using a Mobile One blend. I noticed about a half a quart of loss between oil changes. Since I switched to Royal Purple, I have no loss even though I am going more miles between changes. Not sure why. Hope this helps.
I have 152,000 miles on my RX and change the oil every 7,500 miles--even though they say you can go longer.