Believe it or not, any lead buildup can gall a mold top. I have tried everything from Graphite, moly spray to dry moly and all oils but the truth is a clean, mold cut right with end mills will leave ripples that act like bearings.
The next thing to gall a mold is a burr and all edges of the sprue plate must be rounded and polished.
One of the best is Rapine Mold Prep but sorry, Ray retired. The ripples I make will hold lubes.
Ray Pine knew molds and asked me if I wanted to buy his business but I was on SS at the time.
Cut almost molten lead and you have a mess and even I will get a small smear of lead once in a while. No oil made will stop smears when glove cutting. let the sprue get harder.
Now if you do get a smear, strike a wood match and blow it out quick, Rub the smear with it and it will take it off. bad might need 2 or 3 matches.
About any oil is a good flux, waxes better, keeps oxygen from the metal so it will tin nicely. You just tin your molds.
I made a test of boolit lubes with a soldering iron on brass and copper. All made a good flux. Why flux a mold?
Alox burns, leaves ash. lube is gone. I swear it burns in the bore too. Why else get smoke and stink? I refuse to use Alox in any form for a boolit lube. The stuff was made for a rust preventative on metal and even fails there when it hardens and water gets under it. Just what nut case decided it was a boolit lube?
But back to the OP's question, synthetic will take more heat. But not all are equal, Stihl synthetic carbon-ed up rings to failure and exhausts to failure in chain saws, blowers, etc. Opti 2 solved it. I think that is Bullplate, blue in color.
You really waste your time to make casting faster. Rapine was micro fine graphite in alcohol. Worked very good. yet it never allowed cutting molten lead. Just a lube to prevent wear.