What is the best synthetic lube for our 6-speeds? How much does it hold and what weight do we use for winter and summer.

 

You should have done some looking in the Fluids and Lubes area before posting this question -- which does not have a difinitive answer. Check out these threads:

Redline or Amsoil???
http://forums.maxima.org/showthread.php?t=464672

Synthetic Oil in 6-speed manual transmission
http://forums.maxima.org/showthread.php?t=466581

How to change Manual Transmission Fluid?
http://forums.maxima.org/showthread.php?t=225766

There are others, but these are the most recent. Check them out, and look before asking the next question.