There are a number of reasons that oil may be low in a vehicle between oil changes. The number one reason is a leak. The second reason would be oil burning. The final reason is called oil consumption/sludging.
Most BMW's start leaking at about 30k miles, depending on what type of oil is used. Your BMW should have a
skid plate covering the underside, making it difficult to find, and also holding this oil until the vehicle is in motion (where the wind from driving will push most of this oil off of the plate and dry it off by the time you park it again). The VW also has a skid plate, and the same could be occuring. The Miata is somewhat covered on the underside (under the filter located on the passenger side) and some have stabilizer bars underneath the oil pan near the drain plug. Some of the newer model years have a skid plate that must be removed to drain the oil.
Oil burning is a completely normal occurance. To what degree determines if there is a problem. Oil is designed to stick to the inside of the cylinder walls where the combustion occurs. It's sprayed under the pistons and as the piston travels downward, the oil from the walls are now exposed to the heat from the combustion. This minute amount of oil is burned off and sent out the exhaust. Over time this amount can add up. If the piston rings are in good condition, the amount of oil burned will not be significant and will not be noticed when the oil level is checked. If the rings are dried and worn out, it will allow more oil to get into the combustion chamber. Extended oil change intervals will make this even more noticible. Failure to maintain the PCV system every 30,000 miles can also contribute to burning oil. You won't necessarily see smoke when your engine is burning oil.
Lastly, is oil consumption, or sludging. When your engine is running, air travels through the air filter, into the throttle body. Air filters are NOT perfect. (Not even performance filters). The impurities from the air, combined with exhaust gasses from the vehicles in front of you in traffic, will stick to the throttle plate and the inside of the throttle body, and eventually to the valves and inside of the cylinder walls. These impurities will mix into the oil (and also get burned off increasing emissions and decreasing performace and fuel economy). These impurities are harder to burn off when the vehicle is driven less, or for shorter distances. These impurities speed up the break down of the oil molecules, causing them to stick together and stick to different critical parts in the engine, such as the cylinder walls, the pistons, etc. When this solid oil is stuck to something ABOVE the oil pan, it will not register on the dipstick anymore, thus making you want to add oil. Extended intervals between oil changes will decrease the oil's ability to withstand heat. A combination of extended intervals plus the buildup of carbon, a natural by product of incomplete combustion, draining into the oil pool, is nearly a recipe for disaster for most mechanics, and for your wallet and the life of your engine. Most oils, synthetic included, can handle about 3,000 miles worth of driving (about five uses per week), or 3 months of driving (which if you don't reach 3,000 miles, it's not enough use to burn off impurities and can cause the piston rings and other gaskets to dry out quicker).
I would recommend using a high mileage oil in all your vehicles. Valvoline has a high mileage synthetic oil. The BMW requires synthetic and both the Mazda and the Volkswagen recommend it. Remove all skid plates and clean them off (A T25 socket and flathead screwdriver will probably be required for the VW). Inspect the oil pans for any signs of leakage and replace or have the oil pan gasket replaced if there are leaks. Check to see if any PCV valves are servicable and if so replace them and continue doing so every 3 years or 30,000 miles. (PCV valves are no more than $10 usually). Clean out the throttle body or have a fuel system cleaning performed on your vehicle. Make sure its a full service that includes cleaning of the fuel injectors, the throttle body, and the intake. This will restore lost horsepower, cure rough idleing, and prevent and slow oil burning. Ususally costs $70-$100. Do this every 36,000 miles. The oil burning and carbon buildup will foul spark plugs faster, so inspect and change all plugs. Platinum is better. Finally, change your oil every 3,000 miles, regardless of your owner's manual or oil change indicators! If you take these steps, you will notice the difference in gas mileage, feel the difference in takeoff, acceleration, and overall horsepower, and emissions are greatly reduced. Check out this page too,
Valvoline Instant Oil Change » Mechanics VETO Extending Oil Changes. I googled "Extending Oil Change Intervals".
You have performance vehicles, take care of them.
Phil
How many quarts of oil in between oil change is normal
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