That seal is the worst. When I replaced the oil cooler hoses in my XC I had to R&R the thermostat several times to try and get it lined up correctly, and I never really got it in there well enough that it didn't leak. I eventually replaced the thermostat with one from a junkyard car because the original was making noise at startup. When I pulled the JY unit, the seal stayed stuck to it so I left it alone, installed it as-is, and it hasn't leaked since. Kinda hokey, but it worked. :D
It does seem like the replacements for that seal are oversized, even from the dealer.