I pinged Steve D'Antonio on best sealant for NPT diesel fuel fittings. He said he'd been using Leak Lock for years.
BTW - on another thread for Defender, some mention was made about Amazon. I've noticed for the last year or so their prices are no longer without question the lowest - pays to shop around. This product is a good example:
$21 at Amazon; it's $11 at Supply House, though shipping is only free >$99. I've used Supply House several times - good vendor. My only complaint is one you hit <ORDER> you cannot retract and shipping is fast, but their return policy is good.
Also, at my cabin in the Colorado mountains, I had the local propane company change the regulators to my place. I noticed they coated the faces of the flared fittings with sealant (Rector 5 in their case).
Peter