Myself and the previous owner of my diesel Westy with AAZ 1.9TD engine
have driven many thousands of miles with the aircooled (DK model code)
transmission with no apparent ill effects.  I switched to the later
(stronger) waterboxer transmission because I like the overall gearing
better with 215/75R15 tires.  With the stock 185R14 tires, I would not
hesitate to run the DK transmission.  FWIW, the waterboxer trans can
be re-geared to an even higher final drive ratio (4.11:1, I think)
than the DK.

Mike