I have a 2004 545i and the dipstick tube o ring is leaking at the sump.I removed the cabin air filter holder and no way can get to bracket holding the tube.Has someone done this engine in car?Thanks .

 

It is impossible to remove that. I have had my hand down between the firewall and engine and I can touch the bolt with my fingertips, but there is no way I could use a tool to remove the bolt.
However, the leak should not be that bad to consider changing it. It actually can't leak because there is no pressurized oil behind that o-ring.

 

I removed the bolt when I did the valve stem seal replacement operation. It was a bear for sure. You must remove the fresh air filters, and especially the plastic piece at the firewall...the lower center piece that transitions the air duct to the filter bases. Hard for me to explain, but hopefully you understand what I am talking about.

There is one torx screw at the upper center and two 8mm sheet metal bolts at both upper sides. Then the piece lifts straight up. This will allow enough clearance for a 10mm 1/4" drive socket and very short extension. A deep socket is too long and a short socket too short. Need to use short with very short (i.e. 1" or so) extension. Then, you can just get your hand contorted enough to barely spin the loose bolt and remove it. Getting it back on requires even more patience.

incidentally, I did all this with valve covers removed, so it would probably be even more difficult with VC installed, if not impossible.