Modern Paint, Old Paint Gun?

I had a painting project this weekend, so I decided I'd fire up my father's old Craftsman bleeder-style paint gun for the first time. It worked well enough with the shellac-based primer (unthinned), but I didn't even try to spray the Glidden Alkyd/Oil Enamel. The enamel was only a little less viscous than molasses, and the instructions on the can said "Do not thin." I figure I probably would have had to cut it about 25 to 30% to get it spraying consistancy.
I know my father used the gun to spray enamels back in the 50's.
Should I ignore the instructions on the paint can and thin it to spraying viscocity, or should I have faith that it will spray at it's original viscocity? I do have a viscocity cup, and it looked like the full-strength enamel was going to take 5 or 10 minutes to drain out of the cup instead of 25 seconds.
Thanks, Ed
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Ed Bailen wrote:

You should thin to the appropriate viscosity. The "do not thin" instruction is CYA for the EPA, it has nothing to do with the chemistry of the coating.

--
--John
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