I have experienced only one complete seal failure in which the seal failed
dramatically and resulted in considerable loss of fluid in a short time.
The O ring at the top of the oil cooler on my 87 Vanagon, on a very cold
morning, failed. I replaced that O ring and just had to replace it again after
 an interval of 11 years. But, there is always the possibility of a seal,
which is normally not under pressure, splitting against a rotating  shaft.
There is the chance of a seal cracking due to age and causing  a
considerable leak.  This could happen at the ends of the push rod  tubes.

Rico Dagastino
Memphis, TN

In a message dated 3/13/2010 12:51:37 A.M. Central Standard Time,
starwagen@GMAIL.COM writes:

A friend  has oil leaks from the seals on her 1.9 L.  Question is, if the
leaks  are small (getting some spray in the engine compartment?) can those
seals  have a sudden, complete failure and really endanger the engine or can
she  limp along for some time and just make sure the oil level is kept  up?