The powder is settling in the hopper of the powder measure with the vibration of operating the machine. You can make an additional baffle from a soda can to reduce this. I have templates for round baffles, but I think your's is square. You just cut the can out to the shape of the inside of the hopper, but about 1.4 times longer than the hopper is wide in the direction perpendicular to where the powder enters the measure from the hopper. Cut a notch for powder flow in the middle of each end of the extra length. Bend that extra length in the middle perpendicular to the extra length direction at not quite 45°. Flex it slightly past 45° to insert it into the hopper about and inch or two from the bottom. This will cut down on the weight settling the powder and tend to keep it constant.
WD-40 could clean the grease, but it tends to dry to a tacky surface that attracts dust, not unlike what you are trying to remove. The mineral spirits soak will work. Brake cleaner or carb cleaner will work. Denatured alcohol will work. The light coat of oil is just rust prevention. Mainly you want it on the outside to keep your fingerprints from rusting the dies. You specifically don't want oil in the powder dispensing tube. Use graphite powder if you think you need lubrication of the expander.
The seating die will build up lubricant from the lubricated cast bullets and need to be recleaned periodically. When it builds up enough, the bullets will can start to seat deeper. A little bit of mineral spirits or carb/choke cleaner on a cotton swab works. You can poke it up inside the die without taking it off the press.
Seating bullets shorter can raise pressure. It's a tough call with lead bullets in small pistol cases because the primer often unseats the bullet out into the chamber before the pressure starts to build. So all you can do back the load down and work it back up if you seat deeper or if you change the brand of primer you are using, just to be sure your pressure doesn't spike. I usually figure out the % case fill under the bullet and try to keep that constant as I seat deeper.
Cleaning cartridges after loading can be done. Manufacturers will always recommend against it because its a variable they can't control. But fellows on other boards have done long experiments on vibrating powder and checking for pressure changes and burn rate changes without finding any. Figure about how much rattling of ammunition goes on in transportation, particularly of military ammo, and you will realize it's not terribly vulnerable.
That said, if I dirty enough rounds to justify cleaning, I usually just take a rag damp with odorless mineral spirits and wipe them off as I box them.
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