I'd have to agree with Labnerd. Unless your bearings are shot and you can't maintain oil pressure, there is no reason to use a heavier oil than what the factory specified. I've read one theory that engines use less oil through the rings with thinner oil because the oil rings can more easily scrape it off. Heavier oil would help to reduce consumption through worn valve guides. I would use a Heavy Duty Engine Oil (HDEO) rather than a
Starburst oil in an older vehicle. Multigrade 30-weight dino HDEOs are usually found as 10W-30 and synthetic 30-weight oils are 5W-30 and 0W-30 because they flow well in cold temperatures. And no, a 0W-30 synthetic HDEO will not be too thin, even in Death Valley.
If you're looking for an owner's manual, you can download them from the
Ford Owner's Manual Downloads site but it only goes back to 1996.
ACCCC Engine Oil Article