What would cause a milky substance in the throttle body.

[Erik the Awful]

First gen RX-7s get the same milky oil in the oil filler necks. It's typically a PCV issue. The solution on RX-7s was to give it an Italian tune-up.

I'd verify the PCV system is hooked up correctly and working, then drive the dog-snot out of it.

 

[PlayingWithTBI]

I'd be more worried about the steam ports than a little milky stuff in the TB.

 

[Schurkey]

"Milky stuff" in the wrong end of the PCV system is likely blow-by and/or a failed PCV system. Could also be an indication of coolant in the oil pan. As such, it could indicate a popped head or intake-manifold gasket; or one or more cracked castings. Or some combination. And as I said, an engine that doesn't reach proper operating temp will make this worse.

Point being, there's too much moisture in the crankcase for whatever reason. Figuring out that reason will be crucial to the service life of the engine. "I" would not merely ignore this "milky stuff".