I only use soapy water on all of my diamond stones (Venev, EP Matrix, Naniwa, Poltava and plated diamond plates included) and it works perfectly fine, I can't be happier the performance with soapy water on my Venev stones. The end result is hair whittling edges. Also makes stone maintenance and cleaning easier. Just a few drops of dish soap mixed in a dropper water bottle and you just draw a line with the soapy water on the stone, and smear it around with your finger. Add more every once in a while, while sharpening.
Just make sure to have a clean, moist rag around while sharpening to wipe your blade clean between swapping stones. Some guys use dry paper towels, but those will on occasion scratch your blade (especially if some stray diamonds make their way between the paper towel and your blade). A moist, clean rag works much better. Just keep using a clean part of it every time you wipe the grit and metal swarf off before moving on.
I have heard of guys using oleic or even mineral oil on only the fine grit Venev stones but I really don't think it's necessary. The problem is, once you put oil on a stone it is forever an oil stone, messy and oily and you can never really stack them on top of any of your other water stones or they'll transfer some oil onto them permanently. Some types of stones excel with oil, but the Venevs really do not need any oil.
For cleaning and conditioning, use a cheap Nagura conditioning stone (like the "Bodrid" stones Gritomatic sells, or the very cheap yellow with blue wrapping "King Nagura" that you can buy on Amazon. Amazon lists it at #8000 grit, but don't pay any attention to that. It's irrelevant anyways, you're just looking for a stone with a soft bond that releases grit easily to use for conditioning/refreshing and cleaning.
The King Nagura works very well. Just hold the stone and Nagura under some running water, and rub a flat part of the small Nagura over the surface of the Venev back and forth and round and round a couple time. You'll see how quickly it becomes squeeky clean and perfectly conditioned again.
These Naguras are very soft and they release their grit easily which then rubs between the stone and the Nagura to clean and condition/refresh the surface of the Venev.
If all else fails, you can also clean them with something like Bar keepers friend, but a Nagura works much better.
As far as flattening is concerned, you won't need to do much of that at all. Maybe only once in a few years if you only do personal sharpening, so don't be too concerned about flattening them. If you do need to eventually flatten them, you can buy some silicon carbide loose grit from Gritomatic and use a pinch of that on a flat surface with a little water to flatten the Venev. But this won't be necessary for quite a while.