I agree with Charlie Tripp. I've owned the same '59-250 for 31 years and live in Western Canada. Have often flown in OAT's as low as -35C and the oil temps appear to have been high enough although I admit this is from the original oil temp gauge which like all the others was clearly furnished by the low bidder. I have just installed a JPI 830 with oil temp readout but have not as yet had the "good" fortune to go flying in beastly cold weather. I will be watching the oil temp readout with great interest and will report any meaningful information.
If working properly the vernatherm should prevent the oil from circulating through the oil cooler before it reaches operating temperature. I have heard anecdotal evidence of the oil congealing in the oil cooler at extremely low temps before the vernatherm valve opened. My guess is that this is a problem that should never occur with the newer synthetic oils with their vastly improved cold pour qualities.
A caution for anyone living within 2-300 miles east of the Continental Divide: Don't EVER cover up your oil cooler unless you have checked the air temperatures aloft. In a "Chinook" situation one commonly encounters warm overrunning air. Here in Southern Alberta it is not uncommon to have surface air temperatures as low as -15C yet find the OAT at +5C just a couple of thousand feet aloft.
Don Ostergard
c-flhv@magtech.ca