We used to use "moly-b" when I was working on helicopters in the Navy, way back when men were men, the sheep were nervous, and the ships made of wood.
I still have a can of grease around somewhere.
Among it's other properties is that it doesn't absorb water, has a high viscosity, and a high tackiness. It's great stuff, but pretty pricy unless you're doing really bad things to your old truck.
Speaking of that, it's not that Moly-b is so bad for your engine, it's that synthetic oil itself can cause problems, depending upon the age and mileage of your beast of burden.
The trouble usually comes about when switching an OLD (high mileage) engine to synthetic is that it can actually CLEAN AWAY the crud and deposits that have built up on seals, causing them to leak. Putting in regular oil after that will also tend to leak out. This is why you sometimes hear about synthetics causing leaks. It's not actually the oil itself that's the problem, but the seal was worn, but the crud prevented the leak.
Of course, you can get the same effect by running your engine too rich (when your eyes water from the exhaust, it's probably too rich), or from using a detergent oil in your engine and not changing it often enough. Personally, I don't like detergent oils, period.
As an general rule-of-thumb, if you've got over about 10k miles on your engine, stick with the oil you've been using. If you do switch to synthetic, don't switch back.
You just know that there's always gonna be somebody out there who'll go "shoot, I switched to Mobil 1 (or similar) at 50,000 miles and never had a lick of problems! That guy don't know what he's talking about!"
The exception neither proves nor disproves the rule (of thumb) is all I'm saying.
My opinion, your mileage may vary.
