I'm just saying that if a thinner oil is used, the synchronizers will take even more a beating. Double clutching will be one of two ways to save what's left of them. The other way is to push clutch in and keep the revs at the next shift point and wait until the ?layshaft? slows down then stick it in the next gear.

 

Double clutching just makes the downshift smoother and makes life easier for the synchros.
Once you get used to it, it becomes automatic.