Shocks are a bit of a beast to service compared to forks. Truthfully we always recommend having a shop do it vs a DIYER because if you get it wrong, it can be catastrophic. With that being said here are a few things for you.
In OEM shocks we use Maxima 3wt full synthetic SHOCK oil. - https://www.maximausa.com/product/sy...g-shock-fluid/
This is REALLY good oil and is very stable at higher temps. And the price is very reasonable.
Nitrogen pressure is 150 PSI. The OEM bladder cap has a rubber membrane that you pierce with a needle. You can convert it to a schrader valve but it's not necessary. If you do want to change it, you would have to replace the bladder cap with a RT one.
If you're planning on changing the valving you'll have to grind the peening off to access the nut, this can be easily messed up if you dont know what you're doing. Here's a few pics to show you what I mean.
If you're going this far, you might as well change out the seals on the sealhead. RT sells just oil and dust seal, we like to use a full rebuild kit that replaces all the o-rings.
And last, there is no "volume". You have to fill the shock and make sure there is absolutely no air in the shock. This is why most shops vacuum pump the shock. This allows the shock to be 100% full of oil, and is less mess.