Fluid Loader is missing it's bucket filter (according to documentation it should have one).
Anyway it would be nice if it had one.
The way I imagine it would work is that the Fluid Loader would only accept given fluid (as in the bucket placed in the filter) from adjacent tank (or other fluid container).
This way some more automation could be done when container converts one type of fluid to another and when you are interested in pumping out only the end result - think Fermenting Barrel from Growthcraft (http://growthcraft.wikispaces.com/Cellar).

Maybe it means that Railcraft should support other mods' fluid.

Well I can put different types of sake and such in the tanks no problem; all works fine. The issue is that when I put the 'young' version the Fluid Loader will suck it from the barrel before it gets to ferment into 'fermented' version.
This makes it very hard to fully automate my production...

Said that I have noticed that I cannot put any fluids from Biomes OP (blood, poisonous water etc.).

So it's different liquid have different nbt tags? Maybe CovertJaguar didn't thought about this long ago. That's special.

Since you can practically replace pipes from other mods with Railcraft carts I didn't bother to add any to my modpack - especially since this would come with loads of other stuff I don't need/would duplicate with other mods that I have!
Also adding pipes would make tank carts only useful on grater distances.
So adding this filter would make Railcraft even more 'standalone' or self suffiecient.

Ah, well it's probably a relatively trivial addition

Not sure about NBT... there is a bucket per liquid and they are separate in sense that if I put fermented sake in the tank it is different from young one so I cannot put it there until the tank is emptied (BTW: a button for resenting tank content would be useful in it's GUI).
The thing is that Liquid Loader does not have any filter slot at all (apart form rail cart filter slots).

So here is how Liquid Loader GUI looks like:

And the Liquid Unloader - now note additional filter slot where I can put the bucket in:

And here is just to show another type of sake fluid:

And here is the wiki content that I am referring:

Additionally, if you put a bucket in the Filter slot of the GUI, they will only fill/empty carts carrying the same liquid as the bucket. If you put a minecart in the cart slot it will fill/empty only carts of that type.

This paragraph should at least be expanded and clarified.

So what I want is such a slot in Liquid Loader but instead of it interacting with cart content it should interact with tank content (that is accept or not accept given tank content into buffer).

The reason the loader doesn't have a filter is because you can easily chose which fluid to pump into it.

There's no mod I know of that add pipes that also don't have some way of filtering fluids.

The un-loader has a filter slot to ensure it ignores carts with the wrong fluids.