I would recommend trying oil before adjusting the lower spring - it's wound pretty tight and if you let it go sproing its a hell of a pain to wind back up, especially if you don't quite know what you are doing - as I didn't.
WD-40 is great for getting the gunk out - unfortunately WD-40 itself dries to a gooey gunk in a week or two.
The best lubricant for most machine work is plain old SAE-30 motor oil (you can use any oil, 5W-50 won't hurt anything), it never goes to gunk, stays pretty much where it is put and provides a good lubrication film even at low velocities. Stay away from 3-in-1 and the like, they turn to goo too - Singer sewing machine oil was OK, but the modern stuff isn't. For gears smear on some Lubriplate (or moly disk-brake lubricant - which doesn't throw oil and sticks where it is put). One exotic lubricant a fellow engineer swears by is a mixture of STP and ATF - he used it in his rail dragster's engine, don't know if you plan on getting your MX running that fast.
