The Purple is pricey. Some are using Tractor hydraulic fluid with good results. Here's an old post with some Info.
http://www.turbobuick.com/forums/transmission-talk/178732-mobile-424-tractor-fluid.html
I use Lucas Semi Syn. in my race car. I use to fill my converter before installing until my tranny guy who builds a ton of tranny's for race cars said don't worry about it. It will fill up as soon as you start the car. Just put a little around the seal surface. He and another guy who builds PG for racers both say you won't do any damage to anything if you don't fill it.
I just had my old converter cut open after 10 years and they said it was still in great shape.
That is great to know. But for peace of mind, I will continue to fill it up.