How old is the donk ? I'm presuming original, so 20 years ?
The Womboat has a Buhk DV-24 of the same vintage and interestingly enough the same two issues.
My mechanic has checked out the thermostat and given the engine a good going over for me, his answer is that these things seem to simply run cold. I know that it is not considered good for an engine to run at extrememely low temps but given that ours is also 20 years old and runs like a charm what can I say ?
Same again yet the engine does not seem to burn any oil. I change the oil every 12 months and despite the slick have never had to top up the oil between changes. We usually run the engine a couple of hours each week so it gets a reasonable amount of use. Earlier in the year we went down to Jervis Bay and beset by calms we had to motor probably half way in each direction. Oil level did not change. If like us the slick is very light I'm guessing it is more likely to be diesel than lubricant. If lubricant and heavy then I fear you may have worn rings probably brought on by wear associated with turning the engine over after longish periods of inactivity.
btw..when I say our slick is very light I mean very light indeed. If we run at anchor
it is never of any concern as it seems to dissipate very quickly. I've certainly never noticed a slick builing up around the boat even in millpond conditions. If it did I'd pull the engine and rebuild. You might want to get a decent marine mechanic to check her out if its a bit beyond very light or if you are using oil.
Not too many Vertues in Sydney. Pretty little boats, I've always liked them but then I am something of a Laurent Giles fan. Where are you moored ?