[SLOWn60]

No harm in adding a squirt of oil for the test but do it without the added oil first. Compare figures. If there's a huge difference; your rings are worn. But a leak down test will tell you where the compression is being lost: out the exhaust if the exhaust valves have a problem, out the carburator if the intake valves have a problem and out the oil filler if the rings have a problem. Keep in mind though: the valves need to be closed on the cylinder you are testing or they will leak and give you a false reading. I'm lousy at search but @Motofool probably has all the links at his fingertips on how to do compression tests etc.

As for cleaning the fuel system: if you can do compression tests and reassemble the bike properly; you should have enough skill to clean the gas tank & lines! You may want to send out your carbs to a reputable service shop like @ducatiman as there are lots of places that say they cleaned the carbs but don't really do a complete and proper job of it which will drive you nuts if you still end up with an issue!
Personally; I'd take a stab at cleaning them myself. There are a ton of videos on how to. If it doesn't work: you could then send them out for pro cleaning.