I was thinking this was the guy with 3 point splitter and blowing multiple orings. I think his issue is high return line back pressure due to the quick coupler sizes.
On a standard engie powered splitter, return line 10-25 psi in nuetral might be typical, 50 psi when cycling in retract (higher flow back from closed end when cylcing the rod side) could be common, but not cause for concern. Higher than that I would start to look at restrictions.
this one, agreed, the spool OD may be damaged. Most 0rings fit in a groove in the body, the spool rides inside. They should never cross an open port. Brake master cylinders do, but that is another system altogether.
O-rings are inch based (usually) or metric based. They are commodity items, with standardized dimensions (usually) according to 'Uniform numbers' like -020, -118, etc. Some digits indicate cross section (standards are nominal 1/16,, 3/32, 1/8, 3/16, 1/4, etc. More often referred to as .070, .103, .139, .210, etc.), and some digit indicate the nominal ID.
Hardware store orings are sometimes neoprene for water service. Won't work for oil.
For petroelum, you want nitrile (Buna N) or viton (for higher temps, not necessary for what you need, but viton would work ok) Nitrile is by far the most common
Hardness: 70 or 90 durometer, either one should be fine at lower pressures.
Are there backup rings on either side of the oring? Some are flat, some are dished to support the oring. Backups prevent the oring from extruding the gap. Cheap valves don't have them. If the oring fits the groove, maybe there were not there in the first place. With backup rings, I'd lean to N70 orings. Without backups, I'd look for N90, but N70 shoud work.
So for example specifiying a -212 N70 oring gives you size dimensions, material and hardness. That description is all any bearing or seal house needs to get the right part. And any vendor almost anywhere in the world works with the same system.
metrics go by ID OD and CS in mm. Materials and hardness are the same as inch based descriptions.
One source for information and dimensions.
http://www.thesealman.com/pages/oring_handbook/pdf_files/epm_oring_handbook.pdf
parker oring handbook is the most commonly used, but I don't have a link for online version.
There is info in the parker book on diagnosing common failures and causes.