Agree course ground walnut shells. For lug screws and inserts I like fine ground can be hard to find. Easier to clean the medium from the slots and threads. Walnut shells do leave a layer of oil. A micro-fiber towel wipe with 99% Iso alcohol gets that off. Polish and wax good to go.
Which tumbler is up to you. Vibrating not the old rolling drum type. I have an old spendy unit that is for polishing gem stones. Small container size but very gentle.
Picked up a Harbour Freight 8lb. for larger bits. Coupon it was like $60. Very aggressive and noisy as hell. The lid/gasket must be tight or dust everywhere. Batch of bass T-rods locked in one spot a rubbed a hole through the crappy plastic tub. Duck tape on the outside mended that. Have to check on it quite a bit. Suspect it will rattle itself into several bits. Three year replacement free.
WD-40 is very busy letting dust adhere before it breaks down :-)
Creighton