I'm not wanting to start a lube war but it's been proven many time, including a great test run here that WD40 is a lousy rust preventative, gums up to varnish over time.
I use the stuff but only to insert the little straw thingy into small places to hose out crude, the blow the WD out with my air compressor, then lube with
recommened good lube products.
WE has it's uses but much better lubes abound.
Don't take anything personally please.
About cleaning a black power arm If using real black soapy water, flush with hot clean water, dry well, lube, check next day to ensure rust isn't
developing.
I like Ballistol & water also.
I've used 33%/33%/33% hydrogen peroxide, Murphy's Oil Saop/90% rubbing alcohol with very good results but always check a day or two later for corrosion.
NEVER use petro lubes in a black powder bore. If you must them use alcohol (90%) to clean the oil out prior to shooting it with black.