6L6 G 6L6 WGT WHAT's the difference

What,s the difference between all the 6L6G , 6L6GT , 6L6WGT and all the other 6L6's . I would like to use some of these but which one,s ?????
The General Electric Essential Characteristics handbook lists them as having the same electrical specifications.

The Tube Substitution handbook says that all the 6L6 types are interchangeable, as well as 5881 and 7581 and in some circuits the 5932 and KT66.

A more precise answer may be available if you describe the equipment you are using this tube in. Another Audiogon member may have real world results to share.
Albert Porter, thanks for the input. I was thinking about asking a old friend of your's and mine, but sense I didn't buy these tubes from him I didn't think it would be nice to ask him. (you know who I mean) I have a Dyna ST-70-C-3 that uses 6L6's and they don't like too much heater current and I was wondering if that was differant with the differant numbers . Well maybe I'll get some differant answers.....thanks Albert ( I sold you some 6SN7's a while ago)