In the past have used mainly Rectorseal #5 but I am out of it. I know I have use the Rectorseal Tru-Blu on some before but looking at their products sheet it isn't recommended for LP gas. I though it was and with everything else that it's recommended for wonder why not for LP. This is for low pressure vapor plumbing.  This will be on a 90 degree  3/4" black iron elbow that will have to be turned to a certain position so it will not be tightened until it stops turning. I know some say with pipe fittings the dope only acts as lubricant and does not really seal.