Gasket/O-ring alternatives

[Dave Conner <conner@cfm.ohio-state.edu>]

I want to seal a leak in the cooling system of  the 5K.  It's at the
adapter for attaching the heater hose at the rear of the engine and is
sealed with an O-ring.   I understand this is a common spot for leaks to
develop in the I-5 engine.

The parts fiche shows this O-ring with P/N 900 351 03 and describes it as
21x3N. 
I'm not sure what this means but I suppose the N indicates the O-ring
material (neoprene?), 3 = thickness (mm) and 21 = diameter (inside?).

Can I replace this with an ordinary household  plumbing neoprene O-ring?

What are the pros/cons of the gasket materials available in a tube?  
   Permatex Form-a-Gasket, LocTite Gasket Eliminator, GE RTV silicone, etc?  
When are these acceptable alternatives and which ones are preferred?  
Can these provide a seal equal to or better than the specified O-ring, or
gasket?
There is quite an array available and I know they are very good products...
when properly used.

I don't like to have to make a special trip to the Audi dealer for a piddly
 O-ring, but I don't want it to leak again either.

 
David Conner  Columbus, OH 
'87 5KS
    '89 100E
          ' 86 4KCSQ