Gary, if you look at every aftermarket adapter for relocating the oil filter they have one common feature - the o-ring seal. Moroso, Mr Gasket, Spectre, etc, etc, etc all use the same design. With all due respect, this is not the only part where holding the gasket/o-ring in proper position while jockeying the part into place is critical, but done every day. In my opinion you seem to have simply had the bad luck of a pinched o-ring, and maybe twice on different vehicles over time. Mike pointed out some very good tips for getting it on right, based on his years of working tight spaces on Harley's, that it seems to me would just about guarantee success unless your adapter is not flat, is cracked, or is a porous casting. Maybe all of the guys on the S10 and Astro forums are the experts. How about replacing that remote cooler adapter with a quality aftermarket unit, like a Moroso??
Good luck with your repair, and I'll go out to the shop.