I put a sunpro oil pressure gauge on my '54 NAA a few weeks ago. At the advice of others, I didn't use the 3/16 (?? ) plastic line that came with the kit, but instead used the copper tubbing. Sunpro sells this as an accesscory pack, should be located on the same isle as the gauge.. they also sell a metric accsy. pack in the same area.. The gauge doesnt care plastic or copper.. it is a compression fit with brass fittings anyway.
The brass copper unions onmine never did leak and I used no thread prep on them... Nice thing about brass... usually you can tighten it enough to seal.
Mines been running every other day for the last few weeks with no leaks.
Soundguy