Unfortunately 10 blocks, 5x a week, twice a day isn't even a blip to warm up the car or charge the battery. In the spectrum of ICE (Internal Combustion Engine) wear from worst to best is:
Barely warming up/short commutes... warmed up, but with stop and go traffic.. warmed up long distance drives.
But this is true for any car (engine/trans/battery), it's just that BMWs always tend to be most sensitive to this equation.
BUT, here's the good news: if you do drive it that little on an annual basis, it might be years before you see the effect of that. Say you only drive it 5k annually, it would take you 10 years to wear the engine/trans down enough to probably notice it. Compare that to someone who drives 15k annually, but in stop and go traffic, by year 3... good luck!!
If you CAN balance it out a bit, with long distance runs (as someone else noted), that would help. But again, not sure how long you're going to keep it, but you probably won't see the effect of it before you'd sell it anyway.
Some common issues of that type of driving (not specific to BMW, but in general) are sludging, carbon buildup, catalytic converter wear (you will have built up a lot of condensation that doesn't have a chance to burn off effectively), transmission issues. The highest wear on an ICE occurs during the warm-up period and figure that's the only time you're ever really driving it. My dad used to warm up his car for 15 minutes before driving it, every morning. Only problem is that it didn't do anything for the trans. Auto manufacturers DO NOT recommending warming up anymore, but just to drive slow/gently until it does. HOWEVER, this is mostly due to EPA regs (can you imagine thousands of cars idling every morning?).
OK, feel free to stop reading now, unless you want to know way more than you need to enjoy the car!
On a side note, regarding carbon build up with DI engines, the fuel never touches the top of the intake valves, rather the injectors pump the fuel directly into the cylinders, where ignition occurs. This leaves the valves to accumulate carbon over time. On non-DI engines, the fuel is sprayed into the top of the intake valves, which then coats the valves, acting as a "cleaner" this is where all those wonderful techron and other additives come into play. Sadly, while it's good for other things, those additives won't help with a carbon build up issue since those additives never touch the "back" of the valves (except for one key part, read on). And of course as the carbon builds up, it can restrict air volume or change air flow profiles into the cylinders which would result in drivability issues.
Truth is, there is no way to avoid it (various degrees of it) on a DI engine, so don't worry about it. Walnut blasting (ie mechanical means) is the only effective solution to longterm carbon build up (but what defines build up and what your tolerances are for same is relative). Even soaking carbon covered valves in numerous chemicals don't actually result in an easy removal of it (it is..... after all.. CARBON--think diamonds!!) and would require the intake be taken apart, so walnut blasting is still a preferred option.
But DI is great for power and mileage (as it's so much more efficient). So now companies are considering a mix of DI and old school Port FI (whereby fuel can still be sprayed ON TOP of the intake valves rather than only INSIDE the cylinders). But that just sounds overly complex to me.
Frequent oil changes are important to alleviate CBU as it ensures the proper timing for camshaft/valve/etc (low oil or bad oil can really mess up timing even by fractions of degrees).
And good quality gas is very important as it allows for a cleaner burn. With bad gas, the ignition in the cylinder can be dirty, resulting in a lot of carbon, as the intake plunges ever so slightly into the cylinder, that carbon can coat the top of the valves. Hence CBU. Unfortunately, additives will do nothing to remove existing CBU on a DI engine. But starting off with good quality gas can help prevent it.
Finally, it's possible to tune ECU (engine management software) so that the ignition occurs exactly at the right time in the valve open/close phase to help restrict carbon blowback onto the valves. But DI engines are always straddling a fine line between efficiency, power and misfires. Throw in possible bad injectors and other N63 quirks and a slightly out of timed engine can have CBU.
One thought to help prevent CBU is driving the car HOT (Wide open throttle, high RPMs, good quality gas, perfectly maintained engine). The idea, some say, is that if the engine is running hot and under WOT, carbon can't as easily stick to the back of the valves. The problem with this is that it again won't help with any existing CBU and will result in a myriad of other parts wearing down or failing--if done at the theoretically needed intervals to make it "effective."
As CBU becomes more an issue, more places will offer a cleaning service and the prices of that will go down. While it generally will only result in a loss of power, it's only when it starts generating codes which result in you not being able to pass a SMOG test, that it becomes a real problem.
