There are quite a few threads touching on filling with oil. The devil of engineering lives in the details. Joe's reply touches on an important one: it is worth emphasising this point:

1. Oils expand a LOT. Silicone oil expands about 0.1%/C, so if your equipment is left in the car in winter and summer, that would be a 70C difference. 7%.

2. Sorry to all scientists and engineers out there, pointing out the obvious, but to contain a 7% expansion requires the same pressure to be applied as to compress the material 7%. For oil across a wide surface, that pressure is absolutely enormous.

Result is unless the unit is designed for oil fill, then expansion will crack the case and allow oil to seep out (or bypass the seals), then as it contracts, it will suck in water (if underwater). Bottom line: oil filling units not designed for oil fill can cause a lot of grief.

To modify a unit for oil filling:
A. To fill SCUBA gear with oil ensure you have either a bellows or a diaphram, to allow the oil to expand. If you have a small unit, like a watch, you can do it by putting in a small air bubble and use a strong case, but for anything bigger the volume of the gas bubble you need is too big, and the pressures too large, for that to work.

B. You can reduce the amount of expansion by filling with plastic beads: the same material as the case, and then add the oil. Plastics compress more easily than oil and give you less volume of oil expansion to have to deal with. However, beads make it harder to fill with oil unless you fill under vacuum (the best way), and harder to drain the oil. You also have to contend with the fact that generally the walls of the housing contract because they are in contact with water, and the contents of the housing expand because there is something in there getting hot: electronics, a motor or whatever.

Cheers,

Alex