I use Havoline 10/40W mineral oil that I buy in Asda, its £8 for 5 litres. I change it every 1500-2000 miles, seems to work fine, and it got me to Hungary and back in the summer with no issues (with a family of 5 in it and loaded to the max). I dont think its worth using expensive oil if you change it so frequently, but I'm sure the next person will say something different. Oil usually generates a lot of debate.
Matt.