Bicycle Mechanics - Lubrication of derailleur idler wheels

These pulleys have a bushing. Should they be lubed with oil or white lithium grease?
Or something else?

There is little load on the bushings so type of lube is not that critical.
In general how often you lube is infinitely more important than what you lube with.

Edit - I have worked on Hundreds of bikes of various age and condition and I would guarantee that even if you never lubed the bushings, the teeth on the pully would still wear out long before the bushing.

Enjoy

I use grease when I rebuild them, and prolink or T-9 or whatever to flush around the outsides during normal upkeep.

I have lubed with lithium grease in the past. Works good but kind of a pain to take them apart to lube. I am using Phil Tenacious oil now, easier to get in there. Lots of folks just use a few drops of chain lube when lubing the chain.

I remove the parts, clean them, and lube with a little grease of whatever flavor is handy. I have had a few brand new derailleur pulley wheels squeal like a stuck pig after only a few months service. The pulley wheels and dust caps form a dirt trap that makes it about impossible to lubricate them without disassembly.

with the gear properly tuned, them things run forever.