I have used nothing but full synthetic since the mid seventies. When I got my IDI I first used Mobil 1 because I had it. I then went to the Walmart stuff then to Rotella T6 a few years ago. After a couple trips to SEMA asking about oil for flat tappet cams, I now use T6 in everything; no cats. And Walmart has it in 2.5 gal size. Oil is just about the cheapest item in total cost of operation. Don't cheap out.
BTW, I just started with a clean 93 turbo, clean cooler, and a normal filter. It took 13 quarts.