But ... skateboard bearings have OIL in them, and oil is ... bad?
I see plans for various spool holders, either for PLA in the open, or for whatever filament in a dry box, that use 608 bearings. Elsewhere, I see warnings not to oil your filament to make it go through the extruder better, because problems going through mean something else is wrong, and it's better to fix the other thing. So, if I use the 608's, will oil leak on the filament, and is this bad?
Typically, oiling a filament would mean to use a vegetable based or non-petroleum type of lubricant, possibly even PTFE (teflon) or silicone. Those materials will not damage PLA filament.
Oiling filament is not the haphazard application of lubricant, however. One drop on the filament sponge guide will last a rather long time and should be given sufficient time to distribute itself in the sponge, helping it along by alternately squeezing and releasing the sponge.
Ball bearings of the type you've described will not have oil, unless otherwise modified by the user/owner. The bearings are packed with grease which will not leak out under normal circumstances. Running the bearings at high speed will cause the grease to thin a bit and perhaps drip or if hot enough will "sludge up." If your bearing grease turns to sludge, the bearing has already gotten hot enough to melt out of your plastic fitting.
For spool holder applications, you can clean the grease from the bearing with a suitable solvent (denatured alcohol, acetone, soap and hot water in a pinch) and expect little impact on the drag. The spools rotate at such slow speeds and under such small load that the bearings alone will work nearly forever. This recommendation is void in dusty environments.
With respect to oiling the filament, it's not a bad idea to have a sponge dust catcher that has no oil just as the filament enters the last open location. My bowden extruder system is nearly enclosed and the sponge sits at the very edge of the spool, while a direct extrusion system would have the sponge at the entry to the extruder gears.
Sounds like an instance of bad advice without any real basis in reality.
Oil is not bad, and generally only bearings specifically being sold for use with skateboards will be oiled. I don't mean all 608 bearings, but rather 608 bearings intended for/marketed to skaters to use with their boards as opposed to general purpose 608 bearings. You can tell if a bearing is oiled vs. packed with grease by spinning it. A greased bearing will stop almost immediately due to the viscosity of the grease. An oiled bearing will spin under its own momentum for a while after you spin it. But, regardless, grease or oil, the chance that it will contaminate filament in any appreciable way is very small. The skin oil on your fingers that gets deposited every time you load the filament is way more significant than accidental bearing leakage, and either way, it doesn't matter.
The advice about not oiling your filament is, well, bad advice. Or perhaps it was simply misunderstood. There is nothing wrong with oiling filament if done properly, and every little thing you can do to make the filament extrude with that much less resistance is a good thing. And it is certainly not 'bad' to oil filament, as no matter what, it is going to improve the situation.
What that advice is trying to say is that if the extruder is having problems extruding, then oiling the filament is not really a solution. It is never actually necessary to oil filament, and it is not something you should rely on to fix a problem, but rather something that can increase performance. So yes, don't try to make a non-working extruder working by oiling filament. That doesn't mean oiling it is bad, or you shouldn't do it, just remember that at best, it can only hide a problem, but it won't fix it.
Despite what the other answer to this question says, you cannot 'oil' filament with PTFE lubricant, or any other lubricant that is not actually oil, and the purpose is most definitely not to lubricate the filament. You also do not need to do it constantly, just once in the while. And it is only useful in all-metal hotends. Hotends where a PTFE tube all the way past the heat break will not be hurt by oiling, but it won't get much benefit either.
So what is the point of oiling if it isn't to lubricate the filament? Well, when certain types of oil (vegetable oil, shortening, canola oil, etc) is heated for a long time at a temperature of 190°C or higher, the fatty acids oxidize and reorganize, or more accurately, polymerize into a new plastic-like layer of molecules. This polymer layer acts as an extremely slick coating that is almost as slippery as a teflon coating. Regardless, it is much lower fiction than bare aluminum or stainless steel - having this coating inside your hotend will lower the drag on the filament that much more, which is always a good thing.
If something about this sounds familiar, you probably already know about it under a different name - seasoning. You know, that thing people often do to cast iron cookware. Only, it isn't actually specific to cast iron - it works just as well with aluminum or stainless steel.
Repeated exposure to hot oil continues to build on this coating, making it more slippery and durable. The polymer coating actually bonds to the metal. And you aren't lubricating anything, that is not the point, you do not need to constantly oil filament. You can lower the extrusion resistance in all metal hot ends by oiling during the first few prints, then oiling a print every once in a while to build up the coating again (which will slowly wear away over time), but it isn't something you really need or should do all the time. And using too much oil during a print can weaken the print's final strength because any excess oil will mix in with the PLA (or whatever it is you're printing with).
One final thing - if you do wish to season your hotend, canola oil anecdotally seems to work the best. Using PTFE lubricant will do nothing, and again, the point of oiling filament is to season the hotend, not to actually act as a lubricant.
The important take away here though, is you don't need to worry about any of this. Just use bearings, don't worry too much about keeping filament clean, just have something to clear off dust and other detritus that might be large enough to clog the nozzle. If you needed to be so vigilant as to worry about oil or grease from bearings, we'd all be using rubber gloves to even touch our filament.