I had a Fiero GT 6-cyl. for three years, 99,900 miles before buying my first MI16. Since I lived in Buffalo, NY and worked weekends in Detroit, I was putting on about 1000 miles/week. I used AMSOIL brand of synthetic oil, changing only the filter every 5,000 miles, and both oil and filter every 20,000 miles. The cost came to about the same as a regular oil & filter change every 5000. No engine problems; no oil burning.
For my previous MI16 (the '89 that died last year with 256,300), and for my current '91 MI16, I use the cheapest oil I can find, changing oil and filter every 5000. Consumers Report says oil is oil. Since I've haven't spent any money on engine repairs in decades, I have to agree.