<VV> transmission oil

[Ken Clark]

I have a '66  500 model with 3 speed trans.  Last fall we had some articles that said to use a Gl-4 because of the additives in the oil.   I'm now ready to change oil and add new oil.  I've heard that amsoil  has the best for the synchronized transmission.  Any need to go to that instead of the GL4?   Also, getting all the old oil sucked out is some of a problem.  How  is the best way to assure all the old oil is out.  How may quarts in there when it's full?  I have a suction gun, but hard to tell if all the old oil is out. Ken