Before we begin, I’d like to note that this is historically one of the most controversial topics in yo-yoing. Take it from @Tvelto:
I don’t know why this is a controversial opinion, but it totally is. This is my unpopular [yo-yo] opinion that has gotten more hate than any opinion I’ve ever stated … ever. Yo-yo lube, and lubing your bearings, is essential. Bearing runs smoother, bearing runs quieter.
“Oh, my yo-yo is dead smooth!” Yeah, not if you don’t lube it, because that bearing is going to rattle. “Oh, there’s vibe!” Maybe just lube the bearing, make it so it doesn’t sound like a truck? Because steel rubbing on steel is not a smooth thing. Sling stainless steel knives against each other. Is that smooth? Did that feel smooth while you did it? No. Put lube in your bearing. Yo-yo lube.
Now, there are some people who will tell you that no lube is required, and the only thing you ever need to do for bearing maintenance is to clean the bearing.
These people are cough wrong. 
However, in the interests of representing “both sides” I will agree that:
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If for some reason you like a super noisy yo-yo, you actually do want a clean bearing with no lube.
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If you are going for absolute maximum spin times, you actually do want a clean bearing with no lube. However, absolute spin time measured in 2+ minutes isn’t particularly useful if you’re actively playing with a yo-yo.
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If you don’t want to mess with anything and your bearing seems OK to you as-is, there’s no reason to go out of your way to do anything at all. If it ain’t broke…
As for me, I massively prefer quiet and smooth, so I personally lube the bearing on every single yo-yo I get, whether the yo-yo is new or not. Why? See for yourself! Compare these videos I made of a bearing before and after lubrication on a relatively new yo-yo with a new bearing, the G2 OC:
On an older yo-yo (YYF Boss) the difference is far more dramatic:
Smoother… quieter… what’s not to love? This even works on ceramic bearings – see the before and after.
How to lube your bearing
We’re talking about unresponsive yo-yos here. If you have a responsive yo-yo that returns on a hand tug, lubrication is way easier – put almost any kind of household 3-in-1 type oil in your bearing and you’re good to go.
Before we begin, some truths:
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You only need a very, very tiny amount of lubricant on a yo-yo bearing.
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The lubricant must be thin lube, not thick. If you put thick lube on a yo-yo bearing, you will have a very bad time. I use trumpet valve oil personally, but do make absolutely sure you are using thin lubricant before you begin.
Gotta go with my man @tvelto who has a solid approach to start with. I believe my method is a bit simpler, but this method is highly recommendable, I’ve done this and I can definitely vouch for it as the extra thorough version:
That is:
- take the bearing off the yo-yo
- (ideally) remove bearing shields
- clean it with pure 100% acetone (no additives)
- put bearing on the teeth of pliers to hold it
- use compressed air to spin out the bearing and blow out the acetone
- put a tiny TINY dab of lubricant on the bearing
- finger flick the bearing to verify it is still spinning smoothly, look for 10+ seconds spin time
- reinstall bearing, reassemble yo-yo
The Hyperspin™ Lubrication method
Because I am a profoundly lazy man and I like to exert minimum effort for maximum reward, I’ve found that you can remove a fair number of these steps and get equivalent results.
All you need to do with my method is unscrew the yo-yo halves:
- keep the bearing on the yo-yo
- don’t bother removing the shield
- don’t bother cleaning the bearing
Now
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Put two BIG drops of thin lubricant in the bearing, and flick it with your finger to spread it around inside the bearing. Spin time should now be weak, maybe a few seconds – that means the lubricant is working.
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Grab a paper towel and dab the top of the bearing to remove any excess lube. Keep it in hand for the next step.
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Use compressed air to spin the bearing on the yo-yo as rapidly as possible – a bunch of excess lubricant will come out as it spins, wipe that away with the paper towel and keep going.
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Keep hyperspinning™ the bearing in both directions and wiping away excess lube until you have at least 6 seconds of finger flick spin time; ideally 12+ seconds.
The advantage of this method is that it’s fast and you don’t need to agonize over how much lube you put in (in fact, more is better with this method), but you must be absolutely sure to spin out all excess lube! This also has the benefit of breaking in the bearing, the equivalent of many of hours of play in a few minutes.
It is possible to have cranky bearings that may need more treatment:
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If the bearing is extra nasty, sometimes I have to repeat this procedure with two more drops of thin lube to push out whatever gunk is in the bearing, but that is rare. This way you don’t have to take it off the yo-yo, though.
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If the bearing is acting extra weird, I will take it off the yo-yo and soak it thin oil overnight. Then take it out, dab it on a paper towel to remove the excess, and put it on the pliers and use compressed air to spin out excess.
Testing for responsiveness
With the yo-yo disassembled, finger flick bearing spin time should be 12+ seconds, at minimum 6 seconds.
With the yo-yo assembled…
- throw a hard sleeper
- pull up strongly on the string
- make sure the string does not rotate around the bearing when you pull up
Be careful, because any string rotation around the bearing when pulling up strongly after a hard sleeper is generally a bad sign. You want the string to stay straight up (vertical) and not rotate at all, any string rotation is indicative of too much resistance in the bearing, so the bearing is pulling the string around itself. If this happens, you need more HYPERSPIN™ to flush out any excess lube.
Now, armed with this knowledge, go forth and lube that yo-yo bearing! 
myself …
