Power steering pump leak

[Stewfix]

Hello, I have a Mk7 2011 and have a leak on the power steering pump. Its where the pipe from the reservoir bolts onto the pump. There's an elbow connector that has one bolt that connects to a rubber pipe to the reservoir. i undid this nut and pull it off and found the o ring in the oil that i caught in a bucket. thinking that i could easily replace said o ring with a suitable replacement, i refitted and filled up. at first i thought it was sound but 4 hrs later there is a slight drip. first question do the o rings have to be direct replacements? i found what i thought was a identical match in my plumbing spares box . second , is there anything i should have put around the o ring ie sealant of some kind??
Many thanks Stu