I have an 06 F-150 5.4L 4x4 that I bought a few months ago and I already installed a BMK-13 on it and so far I haven't had any problems with the system. It increased my oil capacity by almost 1.5 quarts and what I especially like is that since I mounted it on the inside part of the drivers side frame, now the oil filter is really easy to get to. Right now I've put almost 1000 miles on the system and I'm planning on running a used oil analysis at 3000 miles then again at 7000 just to make sure everything is ok.
Another reason that I went with the BMK-13 over the BMK-11 is that I'm not very fond of having to make holes in my oil pan for the return hose. This and the fact that it would have been much more difficult for me to install the BMK-11 because I have no way of reaching my oil pressure sensor since there is another differential sitting under my engine. So keep all of those things in mind before you make your purchase because with the BMK-13 I only had to run 10" of hose each way.
One more thing, if you do decide to buy the BMK-13 and don't want to use the Amsoil filters you can do what I did. I crossed referenced the equivalent of the EAO-15 filter which is the second to largest you'll be able to use and instead I'm using a PureOne PL30001. They only cost me about $5.50 and pretty much work just as good for my intended OCI's.
[ September 07, 2006, 11:52 PM: Message edited by: chico80x ]