This bearing adjustment technique reminds me of adjusting the headset on a bicycle. You want to get rid of the play, but not overtighten it. So you have to find that sweet spot where play just disappeared, and not tighten past that point. (Some bike headset use angular contact bearing, in this case tightening a bit more is beneficial since ACBs work well when they have some preload). Either way, when you're done, there shouldn't be any play, but the rotation/movement should be smooth and without binding.
Grease can last longer and stays put better than oil, but this is an open bearing (no seals) and will collect dust/dirt relatively quickly, I'd say it should be cleaned/serviced quite regularly, so oil (same as in your wheel bearings, e.g. Bones Speed Cream) should also be fine.
I just did a quick check, in Germany you can buy 10 sets of 51100+51101 for 16 EUR + 3 EUR shipping. So, 20 EUR for 10 sets while one set from YOW costs 25 EUR. Unlike the wheel bearings, this is not some kind of high-speed bearing where super high precision and quality will result in a better ride. At least I don't think anyone would notice a difference between a no-name bearing and the original YOW one or the SKF. Btw, a set of SKF 51100+51101 should cost no more than 40 EUR.
Why on earth does the manufacturer not put out this video on basic maintenance themselves? And why don't they provide adequate grease in the first place? Brand new surfskates should not start squeaking in the first few weeks! Talking to you @YOWSurf. 82 videos, and zero on maintenance.
