If you are talking about sealing the threads, I know for a fact that 3M Firestop (red caulk) has been used. Pipe dope isn't going to work and I've never seen high temp PTFE paste. What ever you use, don't touch the sniffer cone and don't get any sealant of any kind on it either.
Now, the best solution would be to order copper Dynotech O2 crush washers (18mm I think) from Summit racing unless you have a NAPA in your area that stocks them....and use anti seize on the threads.
How did i know that you were gonna reply to this post? It is welcomed, Thank you!
I did end up getting the sensor to seal using stuff I had laying around the garage. I did put anti seize on the sensors before, but apparently it had caked up and caused an uneven condition not ideal for sealing. I gently used a brass bristle wire brush and cleaned off all the old anti seize from the threads, and the sealing washer as best as i could. I used a scotch brite pad and some emory cloth to clean up the header o2 bung sealing surface and then wiped everything down with isopropyl alcohol. I didn't apply any anti seize this time I installed thinking that there was plenty of compound left in the threaded bung to do the job...time will tell.
Anyway, I installed the sensor and torqued to 35 ft lbs, per my maintenance manual. Cranked up the shop vac and there are no more leaks from the o2 sensor. Problem solved. I ran the truck to the store and back and still had a lean condition. Got off the horn with the cajun tuner guru Joe and he advised reflashing/clearing memory. All seems well and ok now.
Nick name, thanks for your expeditious reply. If the thorough cleaning didn't work, a trip to Napa would have been next. Thanks, Dan