I dealt with this issue not too long ago. The struts need to be replaced in pairs, so if you only replaced one get another new one and replace the other old one as well. Aside from that, I would never buy struts from anyone that doesn't advertise or designate them as THE parts for my 1985 745t.
If you want to compare parts numbers I would suggest you take a look at the lists provided by the two sites below. They are complete, offer cheap prices, and allow you to see all the parts for your SPECIFIC volvo year/make:
http://www.thepartsbin.com
http://www.importeccatalog.com
As for the cold weather, I know that even my new structs have trrouble in extreme cold. I was just in 10 degree (f) weather, and they went up very slowly. However, fully extended they did hold the door up.
Good luck,
Rick
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