Movement oil plays an important role in every mechanical movement- it acts as lubricant that reduces the friction between contact surfaces of the different small components within.
Without the lubricant, the friction created will wear out the movement and adversely affect its accuracy. Any surface contains irregularities, even when polished to a mirror finish.
When two surfaces are gently brought together, only some points on the surfaces make contact and wear against each other. With lubrication, a protective film present on each of the surfaces separates the two surfaces. The protective film adheres to each surface and must resist being sheared off or pushed aside by the movement of the surfaces, particularly under a load.
Today's commonly available lubricants are manufactured from petroleum and consist of chains of carbon and hydrogen called alkanes. An oil molecule usually has between 15 to 20 carbon atoms, while a grease molecule will have 20 to 25.
The side of the hydrogen atoms facing away from the carbon atoms has a slight positive charge, attracting it to the sea of electrons of the metals. This force will of course be considerably stronger in a longer carbon chain. The lubricant adheres to the surface yet easily slips over other lubricant molecules, reducing the level of friction as lubricant molecules rub together rather than brass and steel surfaces.
A lubricant with a longer carbon chain is less volatile, that is, it does not evaporate as easily. As the temperature rises, vibrations break apart the lubricant molecules, leaving a watch movement vulnerable to damage. Since molecules with longer carbon chains vibrate less at a given temperature, they are less likely to evaporate.
On the other hand, we must consider what happens to a lubricant in colder temperatures. In this case, the weight of longer carbon chains is no longer a benefit. Oils with higher molecular weights will thicken more rapidly and clog a watch movement.
In practice, manufacturers blend oils to modify and perfect the lubricating and thickness properties of the combined mixture. When considering how thick a lubricant to use, one must determine the thickness of that lubricant at the coldest temperature the timepiece will be subjected to.
Watches worn on the wrist under normal conditions should not be lubricated with the ultra-thin lubricants necessary in extremely cold environments.
By blending watch oils, a talented watchmaker can achieve perfect lubrication in all temperature ranges.
Our researchers at Ball Watch Company have engineered timepieces with special lubrication for expeditions in the North and South Poles. These timepieces feature specially blended Swiss watch oils that give the watches an operating temperature range from -40°C to 60°C. This special feature allows the Engineer Hydrocarbon to fulfill its promise as a dependable partner for Richard Limeburner in his search for the USS Alligator under the icy ocean.
(From Ball web site)
As for our watches, there is different kind of oil being used at different area, the amount of oil used is critical, too much will cause overflow and splash onto the coil spring, that will cause the watch to be inaccurate, or too less will have in sufficient lubrication.
We have seen a trend of moving from organic oil to synthetic oil which will not tends to get thickened and will not cause the watch to get losing time. That was the indication or alert for you sending your watch to cleaning.
That may be wise to send it to service regularly, from experience if you use the watch everyday, 5 years
If you don't use it at all, I suggest you to take it out and use it a couple of days every 3 to 6 months.
Some people put the watch on watch winder even though he is not going to use it for years, it definitely will cause more wear and tear on the parts, in my opinion, I will not do so to my watches.
Rolex recommends us to service the watch once every 5 years, the price is about HKD$1,200. Usually they will ask for other parts changed such as o rings, or even polish, the total bill will be about HKD$2,600 if you agree the requested extra services. Time, about 2 weeks.
Reference for Rolex Lubrication
HK Watch Fever