Posted by mano 
Is there ever a problem sharpening German knives on Japanese stones?
"Ever?" Well... I suppose there's some particular combination of German knife and synthetic Japanese water stone which won't work. But as a general rule "no problemo."
I thought I read somewhere it was best to use Arkansas stones with honing oil
There are a variety of oil stones, natural and synthetic. Arkansas stones are natural stones mined in a particular area and consist of a novaculite abrasive (a type of chert), held in a sedimentary substrate. Novaculite crystals are one size, pretty much. That means that Arks aren't classified according to "grit" or "screen" size. Rather, the various classifications of Arks depend on the concentration of novaculite within the substrate and the color and density of the substrate.
Because they're natural stones, the quality of any given Arkansas stone depends on the quality of the vein from which it was mined. Nearly all of the best deposits are either played out or nearly so. That means picking a good source is very important. The best names are Hall's Pro Edge, Dan's, and of course Norton.
In my experience, edges sharpened on natural stones tend to outlast those sharpened on synthetic stones. I believe (but don't know) that has to do with the relative complexity of the scratch pattern.
Many modern sharpeners use oil stones without any oil at all. We either use some sort of substitute like soapy water, straight water, or just sharpen dry. For fastest, cleanest results dry is best and it's my preferred method. However, dry stones load up very quickly and need a lot of cleaning.
I have a lot of experience with all sorts of oil and water stones, both natural and synthetic. When it comes to my softer, tougher knives I prefer freehand sharpening with Norton India stones (synthetic, aluminum-oxide, oil stones) for profiling and drawing the first burr, then a Hall's Soft Arkansas followed by a Hall's Surgical Black Arkansas for chasing the burr and polishing. It's a close call between either of my good, synthetic, Japanese water stone sets.
A lot of words to say: You pick'em.
BDL