What is the preferred sealant to use on the exhuast stud threads to prevent leakage?

I coat the stud threads with orange silicon.

any rtv or sealant, just let it setup overnight before adding coolant to the mix... IMO

I've used teflon TAPE not teflon in a tube

teflon is a thread lubricant, to help pipe threads achieve proper torque. It may seal , it may not, its not designed to seal threads.

I agree on the teflon tape, but make sure not to get too crazy with it. You do not want pieces of it floating around in the coolant passages.

Use a high temp anti seize on the nut end of the studs, especially if stainless, to prevent any galling.

The studs have a shoulder to do the sealing but I'd smear a bit of silicone on there.

I agree that the pipe thread stuff MAY work but that's not it's intended purpose. Tape and paste are for tapered threads.

Sealing a straight thread was never meant to be done.

permatex #2 in the toothpaste like tube & has about the same consistency of toothpaste

I use "right Stuff"... or is it Great Stuff?

Either way it comes in a caulk tube from the local auto store. It's a bit pricey, but works great for any type of gasket surface. I use it on everything.

But a dolup on the bolt smear it around, torque to spec and don't touch it for 24 hrs. after that retorque and its good to go.

Right Stuff is what I have used in the past also.

I went ahead and used the Permatex #2. Thanks for your input guys.