I was wondering, if everything is working correctly in your engine/car, is there still a way that you could over time lose fluid? If everything is good should you never need to add say antifreeze or oil or other fluids or do over time all fluids need to be a bit refreshed?
To answer your questions:
- Yes.
- Replace all fluids according to the owners manual. Check and top off all fluids (air in your tires counts as a fluid) periodically (at least once a month or at every fill-up of the gas tank.)
Coolant is usually 50% water, and water evaporates. Antifreeze also evaporates, but at a different rate from water (not sure if slower or faster). So you will lose coolant slowly from the coolant reservoir through evaporation. You can also lose coolant (quickly) if your cooling system develops a leak, or if an engine gasket fails and the engine begins to suck in coolant. Some leaks are visible, some leaks (internal) are not. So it’s a good idea to check your coolant once a month or so and top up as needed. If coolant is suddenly disappearing invisibly, that’s usually a sign that something bad is happening inside the engine.
As for oil, some engines burn oil. So it’s also a good idea to check your oil level regularly.
I think you already know about transmission fluid 
All fluids should be periodically refreshed except perhaps differential fluid in a RWD car and manual trannys.
Oil disappears because a fine film of it is left in the imperfections of the cylinder walls after the wiper rings come down in order to provide lubrication for the compression rings, and that oil gets burned in the cylinders and sent out the tailpipe. And it’s subjected to dilution from blowby gasses, particulates from wear, heat damage, and shear damage. It needs regular refreshing.
With the exception of the vent hole in the overflow bottle cap, the cooling system is a closed system, and while the venthole is tiny, water would eventually evaporate out, albiet very, very slowly. But coolant should be replenished at least every 5 years (or sooner) because the corrosion inhibitors become depleted, and because it’s lubricating a mechanical movement, the water pump, that can introduce a small amount of wear residue…plastic impellars can even erode away and become suspended in the coolant.
Brake fluids are subject to contamination from the internal moving parts, like the piston seals in the master cylinder and the seals in the rest of the system, as well as well as being able ot absorb moisture through the vent in the reservoire. Every time you push the pedal you push fluid out the MC, it pulls fluid from the reservoire, the level drops and pulls air in through the vents, and another opportunity for moisture or contamination occurs. I won’t bother to get into the contamination from the “square O-rings” that seal the cylinder pistons.
Autiomatic tranny fluid is subjected to numerous seals as well as heat and sloshing around in the torque converter. It being a hydraulic fluid controlled by valves and controlling solenoids, it needs to be occasionally refreshed.
Even the water in yout battery is subject to refreshing.
In most cars, some oil needs to be added between changes.
How much you need to add, and how often you need to add it will vary, with as much as 1 qt per 1,000 miles being considered “normal”, even for a new car.
Unless you have a truly maintenance-free battery, you will also need to add water (distilled, please!) to the battery on an occasional basis.
Your other fluids only need to be replenished if there is a leak. In other words, while engines will normally consume some oil, they do not consume coolant, and transmissions do not consume transmission fluid. Similarly, differentials do not consume differential fluid. So, if you need to replenish those fluids, you have a leak.
Um, I guess I shouldn’t use the words “refresh” and “replenish” to both mean “drain and refill (or flush) with fresh fluid”. The meaning seemed obvious when I wrote it, but I guess it perhaps is not.
Good point about the battery. Even most so-called “maintenance free” batteries have caps that can and should be removed and the level checked, and only distilled water should be used.
Small decreases in fluids that result in a need to top off can be normal. As brake pads wear the level of brake fluid drops slightly. Some small loss of coolant is normal, and you might need to add an inche or two to the coolant tank over say 6 months. It isn’t out of the norm for a motor to need some oil top off between oil changes.
Differential, transmission, transfer case, and power steering fluids should stay the same, spotting on the floor or drops in fluids are due to leakage and should be investigated for repair.
There should be no fluid loss in closed or closed loop systems. That’s what the word closed means. These include the brake, cooling, power steering, AC, and automatic transmission systems. If there’s no leaks in these systems the fluid levels shouldn’t drop.
Then there’s the vented systems. These included engines, differentials, manual transmissions, transfer cases… These are vented to atmosphere so as the components and oil inside get hot, the pressure this creates doesn’t blow out the seals. So these systems should be checked for proper level as some of the oil is lost thru the vent system when it turns into a vapor.
Then there’s the consumable fluid. Check your washer bottle level?
Tester
"There should be no fluid loss in closed or closed loop systems. That’s what the word closed means. These include the brake, cooling, power steering, AC, and automatic transmission systems. If there’s no leaks in these systems the fluid levels shouldn’t drop. "
Does this include coolant when there’s an overflow reservoir? In my last few cars, they never seemed particularly air tight. The cap just clicks on and isn’t a screw top or anything. It’s definitely not pressurized.