Synchros will work less effectively if the oil is too viscous. Double de-clutching is a good way to overcome the issue (easy with practise).
May I ask why you are so averse to being given directions to use the search function? Do you think that someone else should search and then hand the answer to you?
The answers to all things automotive have been published a million times in as many publications. Most of my knowledge (precious little that there is) has been gleaned from reading and having a go and asking people who are in the know.