[majakdragon]

If you really need to dryfit, I would suggest the K-Y. It is water soluble and could be washed off before permanent installation. Oils and vaseline would/could leave residue that will interfere with adhesion. Best is still to measure the pipe and fittings.

 

[majakdragon]

Since it not a question of length dimensions but angles, you should be able to slip the pipe and fittings in far enough without any lube and then mark the positions on the side or the pipe and fittings with a Sharpie marker for assembly. If this doesn't work, go with the K-Y. Good luck on your project.