Which fluid is leaking??

Not if it's really old or slightly burnt. Transmission fluid would be the same.

Could be water from the cooling system.

Colours

Oil - translucent amber (typical new) to black (old) - artificial oils may be coloured. Water - clear, can be coloured with anti-freeze (blue / green ...) and gets brown with rust after a while. Transmission & PSU fluid - pink getting grey as it ages. Brakes - thin translucent and yellowy.

Well it sure >looks< like dirty engine oil (dark amber with some black streaks around the edges of the puddles), but I'm at a loss as to how the car could dump such a quantity in a matter of hours. Being in the city, it's unlikely the guy ripped open his sump on anything (but never say never).

That's why I assume it could be a leak in the steering reservoir - this seems like a BMW weak spot.

If it IS engine oil (still not sure), what sort of event could cause a BM to start shedding large amounts of it in that manner?

Probably engine oil. Oil leaks on old bimmers are not all that rare. The valve cover gasket design (especially on the V8's) is wont to harden (it's made of rubber) and allow seepage to occur which pools in certain places so it can dribble out later on the driveway. Don't ask me how I know this... ;-)

Most older bimmers used a variety of Pentosin hydraulic fluid for power steering fluid which is clear or greenish color. Transmission fluid is either clear or red on the way in, but who knows what color after 100k miles...

-Fred W

This amount of fluid should cause a noticable level fall in whatever is leaking.

Another thing that might leak is washer fluid, if the non return valves fail the fluid can just run out via the headlight washers. Normally clear or blue (if you put in deicer) though.

I once dropped my 735i off at a park and fly outfit at Heathrow airport.

On my return, they sent a bloke to get it. He must have thrashed it from stone cold, because I noticed a track of oil ending underneath it when they brought it out. I noticed another track of oil when I reversed to turn around. Looking underneath I noticed oil pouring out of the bottom. I called the AA. They discovered that the seal on the filter had blown out, so they put it back in place, screwed it up tight and put some more oil in. I then drove it gingerly to Altwood in Slough, where I was able to call in a favour (my service guy had been promoted to manager) and get it on a ramp late on a Friday afternoon! No problem and the car went on for may years and miles.

More recently I had a small leak on a 740i. This was because the filter top wasn't quite screwed up tight.

Not necessarily. Power Steering Fluid and Automatic Transmission Fluid were once the same thing, but that is not always the case these days. But, yes if it is the same thing, it should be red. UNLESS it is very old and poorly maintained, then it will turn brown.

The A/C will drop a large puddle of water, and if the garage floor was dirty, then the oil on the floor will cause it to look ugly ...

Nah, it's definitely not water. For one thing, none of it has evaporated in the last 48 hrs - still a huge puddle.

Had another look - it actualy does have a slight reddish tint to it, but it still looks like engine oil.

I forgot about AC condensate.

Dip the engine and see a) what the level is (could have been overfilled and be coming out of the labyrinth) and b) what colour it is.

If it is not engine oil and it is pink (blot some with white paper) then the quantity suggests that either all the PSU fluid has run out due to a blown hose or similar (power steering will not work and the brakes will be very heavy), however the car will be drivable to repair place OR the auto gearbox has a major leak (it is auto isn't it?). In this case the torque converter will not work, the car should not be driven (or you will quickly burn out bands etc.) and should be towed off for repair.

You should also look for where it is coming from. Rear of the engine low down implies gearbox. Front of the engine from a pump with a fan belt or the steering rack implies PSU.

How old is "older BMW"? Some BMWs that are only a few years old have been known to get cracked expansion tanks.

It's not his car. He is wondering if a leaking BMW is normal.