Cliff,
My 1977 P337 has oil/air separators as stock equipment - they look very much like units offered by M20.
I have no before & after experience with the 337, but I have tried both the Walker (now Wolfe) and M20 oil air separators on my T210N. The bottom line is they both work as advertised. They greatly reduced the oil on the belly of the 210.
The pros and cons of each are open to debate. The Walker unit requires a connection to your vacuum system, and it’s argued that this allows warm air into the gyros after the engine stops. Theoretically this promotes condensation inside the gyros. Finding room for this unit under a 337 cowl may be a challenge. My shop damaged the Walker unit at one point, and we had lots of trouble getting the seal between the two halves to stop leaking oil. Walker was out of business at the time, so I ended up switching to the M-20 unit. By the way, the M-20 is a one piece device – gasket-less.
The M-20 unit is smaller and requires no vacuum connection. However, it took me about a year to get it working correctly. Bill Sandman from M-20 was always very willing to help. My shop had not installed the unit correctly, and despite phone calls to M-20, they could not see their mistake. As soon as I provided a digital photo to M-20, they were immediately able to explain what we did wrong – and once corrected, it worked perfectly. In a nutshell, oil flows downhill and the separator must be higher than the tube that returns the oil to the engine – and the return tube must not be below the oil line in the valve cover.
I recommend both units – but I have a slight preference for the M-20 due to its size and lack of connection to the vacuum system.
There’s an article at AvWeb on the M-20:
http://www.avweb.com/articles/separator/
M-20’s website:
http://www.m-20turbos.com/breather.htm
Walker’s website:
http://www.airwolf.com/Products-WalkerAIRSEPS.htm