Had small leaks on my tractor quick connect hydraulic fittings.
Several years ago I bought one of those Universal Oring replacement kits.
Probably has 100 Orings in it.
So removed the old hard Orings and installed same size from the kit after
checking required size off manufacturer website.

With newly installed Orings the leaks continued

After some research I thought the problem might have been wear on the
quick connect couplers. So replaced those.

The leaks continued.

Next time I was walking around HDepot I grabbed same size Orings out of the
plumbing repair section. When I got home researched the Oring manufacturer
website to make sure they were suitable for pressure and hydraulic fluid- they
are. So installed them in the quick connect couplers.

The leaks stopped immediately!

Lesson learned- Orings sitting around for more than 2 years should probably be
thrown in the trash. Even tho I stored them in a dark cool place and they felt and appeared in good shape they had apparently shrunk or deformed or whatever.

How did you check the size of your 2 yr old rings, in some instances, ther's a small difference in ring section dimeter. Some standards are 2 Dash series: .070, .103, .139, .210, & .275.
Thern there's the ones diesgnated for ports 3 Dash series .185, .239, .301, .351, .414
So make sure you measure carefully

My experience with the universal kits has been that the oring material is smaller in cross-section.
Ie., they don't meet the SAE standard for o-ring dimension.

We have Princess Auto here which is very much like Harbour Freight, they may be even associated. Princess Auto sells hydraulic equipment parts as well.
The universal O-ring kits are cheap junk, but if you get the correct O-ring in the hydraulic section they are a much higher quality. They also cost a bit more.

The kit has the id and od listed for each Oring.

now.

Yes, cross section diameter is different depending on application. Those QC's can have SAE threads, (which is o-ring boss, same thread as JIC) or British standard pipe parallel. The BSPP fittings work with an O-ring, bonded washer or metal sealing washer. IIRC the cross section for the BSPP o-ring is bigger. obviously the thread doesn't matter in regards to it sealing correctly but the sealing area is different from ORB to BSPP. The universal kits you buy should give you, ORB, flange, and flat face (ORFS) o-rings. All with different cross section diameters. Sounds like you've got this leak figured. But one of those kits is real good to have for those never ending hydraulic repairs.