Lube It Before You Assemble It
Never Never Never assemble AN fittings without lubricant!
Doing so will cause them to gall or seize together so that you must destroy the threads in order to separate the parts
You can use oil to lube or even anti seize, but both of these lubricants can't handle the loads that a precision threaded part can develop during assembly
Fragola's Assembly Lube is purpose formulated with high pressure additives to prevent thread galling and ensure that the proper thread engagement is maintained.
Sold in a 6 oz flip top bottle