FM, from what I can see your stone looks like my Norton. If it is similar to my Norton :
The rough side of the stone ranges from useless to damaging. At least for softer metals. Good for a ax, not so much for cutlery unless doing a massive reshaping of the blade. Scratches the crap out of about anything it touches.
To clean the gunk I would soak the stone in mineral spirits/ Naphtha. That will also pull some of the oil out of the stone, no big deal.
This stone is bad assed hard. I wouldn't worry about it being flat. If you eyeball it and it looks straight, or you can't feel a dip, you should be fine. About the only way to check this for straigntness is by looking for daylight under and feeler gaging against a steel rule ... or spotting it against a granite table. But really, you aren't polishing plugs against a machine way, it would have to be pretty bad before being unusable.
Far as flattening, about the only way your going to wear this stone away is against another stone. I believe these stones made of alumina which is one step softer than diamond.
You take two similar stones, wet them with a light lube like mineral spirits then rub them together. Rinse with spirits, turn the stones 180 degrees and rub them together, rinse, rotate and rub, etc etc etc. With patience and care the stones will become very flat. If done with three stones they can become flatter than flat. In theory immeasurably flat.
Fact, we use to make our standards by spotting standards together, scraping the high spots, rotating, spotting and scraping. Etc, etc, etc ...