[HoodStrype]

A) Royal Purple may be thinner?
B) Check for a crack in the transmission case close to the tail. (Ripping thru gears will crack the stock tranny- I've done it) If so she's kaput.
C) Smoke is actually Antifreeze probably. Shouldn't do it with the heat turned off. It means your heater core is bad. If that's the case, join the rest of us with no heat- it's a 400 - 500 dollar job! That is, unless you want to drop the steering wheel, and take the entire dash apart... then it's a $25 job.

I promise you, I've had 3 Mustangs, and all have had the same heater core problem. If you can go without heat, take the 2 heater hoses going into the firewall behind and slightly below the Throttle Body, and disconnect them. Then just connect them together. That'll bypass the heater core til you can get fixed. Good Luck!