i thought this might be of some interest to those of us with hydraulic
valve lifters:

from Road & Track, February 1996.
Technical Correspondence
...
Heavy Lifting
I own three mid-Eighties' VW/Audi products (a 1986 Jetta Turbodiesel,
1986 Audi 5000CS Turbo and 1985 Vanagon Camper) with hydraulic valve
lifters. If any of htese cars sit for more than a week or two, it's a
good bet that a valve lifter will stick when the car is started. When
this happens, it takes anywhere from seven to 10 miles before the
lifter quiets down. All these vehicles have between 70,000 and 80,000
miles on them, and all get regular oil and filter changes at 2500-
3000-mile intervals. What causes this problem, and how can I avoid it?
                        Bruce J. Villere
                        Sierra Vista, Arizona

Deposit buildup in any engine is a normal phenomenon, and you've
already taken the right steps to reduce it with your frequent oil
changes. But a hydraulic lifter is a device with very closely
machined tolerances, and 70,000-80,000 miles of operation -
especially where the vehciles are not driven for periods of a few
weeks - do take their toll. The lifters get gunked up to the point
where the engine's oil pressure isn't enough to fully extend them
to take up the valve lash - that is, until engine heat assists the
process.

You might try an oil additive formulated to free up sticky lifters
or simply try starting and running your vehicles on a more frequent
basis. And remember, the ticking sound that results from a sticky
lifter is really more of an annoyance than a real threat of
significantly reduced engine life.