First question is the U-turn legal? If not and you have a prang while doing it, you could be in a lot of bother!
Second is have you tried double-declutching? Having done a bit of checking it appears you have the right or at least very similar oil to Volvo 1161423 (the other one is Redline MTL and a few others i found that are directly usable) and they are all 75W rated, GL4 lubes.
In simple terms, with an oil that thick, when it's cold it's going to slow down the synchro rings making engagement difficult. Hence the double-declutching suggestion. Takes practice to get it right but it might offer a solution.
The other would be to move or find an alternative route.
Slightly tongue-in-cheek but i think you're somewhere between a rock and a hard place on this one.
