I have always used engine oil on the basis that I would always have some spare oil on board if I needed to top up the reservoir. (To support my theory an article in PBO a few months back suggested the same thing.)
For the reservoir connection I used an old (but un-used I might add) hard plastic toilet connector with an insert rubber seal. Fitted with the wide (seal) end at the top it makes a fine reservoir that I sealed with a disc of flat and chamfered GRP held in place with a couple of rubber bands.
A year in and no problems so far - not even a need to top it up.
the type of oil isn't too critical as long as it doesn't attack the transducer material. (speed of sound through oils varies very little from one type to the next and as the distance travelled is so small you'd never measure it!) veggie oil can be used and could be a good idea from a polution point of view if it leaked into the bilge and was then pumped out. (its biodegradable.)
Castor Oil is the recc'd oil to use and is available from any good chemist. Normally in 50 / 125 / 250ml bottles. I normally buy 2 small bottles when installing E/S and keep one as back up sealed. So some points that I can pass on, you probably already know but some may like them !
1. It is only necessary to put about 1/3 inch in the tube, in fact a lot less can work, it provides an 'airless' face between transducer and hull surface. Overfilling only makes a mess when you extract to check the transducer. 2. When inserting the transducer into the tube, push down slowly and steadily, DO NOT force it too quickly as it will airate the oil and it will take time for the bubbles to dissipate, causing delay in its ability to transpond. 3. When fixing the tube to the hull, or removing and re-fixing ..... a)wipe away as much 'gunge' as possible with kitchen roll etc. Then take Mr.Muscle or similar Oven Cleaner and spray area and allow to foam amd stand for about 10 minutes. Then wipe it away and then wash off with fresh water. Dry of the area and you will find the bonding area clean for bonding with Glass fibre resin and Micro-balloons / filler.
Many tubes are glassed in and 'weep' due to bad bonding .... look around your tube and watch for sticky 'black' stains coming away from the bonded area - thats a leak ! Follow the above with oven cleaner and good bonding and then you should find the oil lasting for years !!
