Dan,

While I have heard rumors of people succesfully mixing a little STP, or
even dry graphite powder, in with the hydraulic fluid, I can not say
from personal experience that these will work just fine and last a long
time...

However, Cessna does sell a "cold weather" seal that replaces the O-ring
in question, as well as the two S1628 backup rings on either side of
it. It is a much sturdier piece, has a sort of squared-off cross
section, and appears to have some teflon-like properties. I *think* the
part # is "A2748", but I'd be sure to double check that before ordering.
Aircraft Spruce lists the part for $17.50

Happy Flying!
Scott Skylane
N92054