So, I'm working on adding BuildCraft to my private mod pack and server. (This should surprise @asiekierka a bit!) So far, my tests are proving to be very positive. EXCEPT... I disabled BuildCraft from doing world gen, because I already have oil deposits from another mod: Artifice. So in my testing, I tried setting up a pump to drain an Artifice oil deposit from the desert into a tank. I tried using a mixture of stuff from my modpack, as well as pure BuildCraft materials. I can't get it to pump. I can pump water fine. I can pump other mod fluids fine. (Tested with MineFactoryReloaded Mob Essense and Thermal Expansion Destabilized Redstone) But for some reason, BuildCraft isn't detecting the oil fluid from Artifice as a fluid. I don't know if this is an Artifice issue or a BuildCraft issue, but something isn't being properly detected here.

@Techokami - This is especially weird as Artifice, MFR and TE I think all use CoFHCore for liquids. Time to debug!

@asiekierka Yeah, I checked the Artifice code and it uses CoFH core for liquids, which is why I tested with other CoFH-brand fluids. Something ain't right here!

Calling people who might have some idea what's going on here: @Shukaro, @skyboy

Might be artifice not associating properly with BC's oil Fluid object or otherwise BC not accepting artifice's oil Fluid.

I think this is related...

Okay, this was really screwy, but I figured it out.
It all started with adding Railcraft to my server pack. I was getting a crash, which was reported as Railcraft/Railcraft#260 as a problem with BuildCraft-related code in Railcraft. There turned out to be two solutions: either have BuildCraft present in the modpack, or rename the Railcraft package so that it loads first. Since I wasn't originally going to have BuildCraft present in my pack, I went with the latter option.
While doing testing for adding BuildCraft to my server pack, I was working in a development copy of the pack (plus a few other mods for Computronics testing). Railcraft still had a renamed package. Artifice oil was not being detected by BuildCraft pumps.
While discussing the bug with @jaquadro (who was working on some Artifice bugfixes and had a dev environment handy), we found that an environment with JUST BuildCraft, Artifice, and CoFHCore did not exhibit the bug. He considered that something was fucky with mod loading order... and then I remembered what I did. I un-renamed the Railcraft package, and now it works.
So the real culprit was Railcraft.