Tor,
It is a desired residue. The residue tells you that the engine as breaking in. Even if the engine is brand new, you can set the needles close to optimal, just not quite as close as when the engine has reached optimal condition. You should run the oil and contents that the manufacturer prescribes. To check if the L needle is set right, I will check the following. Does the engine have to warm up before it will take the off idle throttle? If yes, that's how I like it on an optimal engine, but not on a brand new. Does the idle get lower with time or does it have to clear exessively when given gas, it's too rich. Set the low, where it will take the gas immediately after starting, but won't take forever to clear out after 10-15 sek of idling. Then set the H needle to where the engine doesn't surge, meaning reaches max rpm, then goes down in rpm, but you probably won't and shouldn't use full throttle for extended amounts of time in the air when breaking in.
Don't feel bad about mentioning "factory needle settings". If the engine runs fine, as you said it does, there is no need to mess with the needles.

DKjens