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If it's the o-ring at the base of the cylinder that is leaking oil, I would only replace if the oil leak is more than a slight drop on the floor occasionally.
Replacing cylinder base gaskets involves removing the motor, tearing down the motor to the case. Yes, it can be done where you only replace one, but the motor still needs to be torn down (ie cam towers, heads, cylinders) on the one bank (4,5,6). All depends on the mileage and condition or the motor to determine how far you go. Being that far in, makes sense to do a "top end" at the same time.
Ask the shop what the leakdown test shows? Also, have the shop run some dye and pinpoint where the leak is. A leak at the o-ring for the case through bolt can also be misdiagnosed as a cylinder base gasket leak.
Also, how long has the car been sitting? What does oil consumption look like? If its a minor leak and the leakdown numbers look good, then congrats on owning an air-cooled porsche. Put something under the car to soak up the oil and drive it.
BTW If the engine has never been rebuilt, here's what @ilko and steve weiner say: Under 5% - awesome Under 10% - good Under 15% - acceptable Over 20% - bad news for your bank account
If all you have is a minor leak from one cylinder base o-ring, just drive the car. Oil is cheap. If it's pouring out oil, that's different. But if it's just a drip, ignore it.
Our 1984 Carrera had a leak for at least 20 years at the base of #6 cylinder. Just an annoying leak, nothing major and it never leaked more than a tiny amount. Just enough so I could see it. I would occasionally wipe it down with a shop rag. My very experienced 911 expert said "leave it alone" If this were my car I would leave it alone. See how it's doing in a year or so. Our 993 has a bit of show of oil at the base of #3 and I am not planning on messing with it. They all leak a bit after a while. What weight oil are you running in it?
If just a minor dripping, drive it and clean it from time to time until a full rebuild is needed / can be afforded. 25 year old car, air-cooled at that, a little oil leak is no big deal, even down on HP/TQ, no big deal. But if a rebuild, then prepare for "while you are in there" and no regrets, all the labor to fix #6, as others have said do al the gaskets / o rings, cause others are sure to follow/fail.
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993 leak in clynder 6
