I'm a bit skeptical about varnishing being a problem if an engine is not regularly started - at least, a comparable problem with the inevitable problems which come from starting an engine from cold, like the inevitable condensation
in the bores.
Admittedly sulphuric acid in that condensate is less problematic in this low-sulphur diesel age, at least in some parts
of the world.
More of an issue (I think) is the irregular and delayed detonation (often incorrectly believed to be premature detonation) caused by the atomised fuel recondensing in a cold engine. Hence the "clatter" which all diesels exhibit until the bores and combustion chambers warm up. The oil
is (as yet) not well distributed through the engine to cope with the severe shock loadings from the irregular detonation.
In a 'gold standard' installation
, the coolant
and the lube oil would both be able to be pre-heated to running temperature, and the respective circulation pumps both be run for a few minutes before starting the engine, and if this was done (and the engine spun a few times with the decompressors open and no fuel being injected), I don't see any impediment to running the engines once a week or fortnight: I think it would be beneficial.
This is just anecdotal, but I've just been present at the inspection
of the injector and associated parts of a single-cylinder marine
diesel engine which has had multiple long periods of inactivity, often not started even once in a year.
The injector was the original, never removed in 20 years. There is no evidence of varnish
Maybe some sources of diesel are (or in former years, were) more prone; I can't comment on this, but I've not personally found varnishing to cause identifiable problems, or had it visibly affect parts I've stripped and checked.
There may admittedly have been injector pump
problems I have been unaware of arising from this cause, though ...