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I live in Canada. On days down to -20C or -30C (-4 to -22F) I find it takes like a second and a half with decent force to move the shifter from 1st to 2nd on my np435. It can be real slow in the cold. I'm running 80w90 and its still like molasses. I wouldn't worry about it really I've never had any issues over a few winters.
GL5 is more 'slippery' which is good for bearing & gear life..........but that same characteristic causes shifting to be harder than with a GL4 oil.
This is because the synchro rings need friction to operate, & is a recognized issue.