If you've been using the same oil since new, and you haven't experienced any obvious "sludge" buildup or oil consumption, then something fundamental has changed. Root cause analysis is needed, then resolution...
"Sludge" is a somewhat generic term for sticky black matrix deposits in the engine.
There are a few types of sludge. One is inorganic sludge. Comes from contamination by fine particles of metal, sand, dust, etc. that are dropping out in slow spots in the engine, forming a sticky, black matrix deposit with oil, or components of the oil.
Another type of sludge is formed when a contaminant causes a chemical or mechanical reaction (an example of a mechanical reaction is a stable oil/water emulsion, looks like chocolate mousse) with the oil or one of it's additives, which overwhelms the additive package in the oil you are using, causing it to fail. This can be caused by localized high heat, water, anti-freeze, or some other contaminant - organic or inorganic.
Deposits can also form on a localized hot spot, as oil can "cook" onto a hot spot and cause a deposit buildup over time - but this type of deposit is usually quite hard.
Whatever the type, if you are seeing sludge, then somthing is overwhelming the dispersant additive in the oil - what causes contaminants to stay in solution so they can be carried along and filtered out in the oil filter. Once the dispersant additive is overwhelmed, it fails, and contaminants - "sludges" - drop out of the oil, and typically can be found in areas of the engine with lower-velocity oil flow, places where oil can form "eddies," or areas of cooler temperature. Of course, it is also possible that there is so much sludge forming in the engine oil that it is also overwhelming the oil filter, causing it to bypass, and cease filtering anything... but I would think that would be rare, without other more serious symptoms. Like an engine siezure...
(Once a contaminant or sludge is present, the tendency for an oil to foam is increased. Once foaming starts, lubrication efficiency drops, and the tendency to form sludges increases... adding to the original problem...)
Do not confuse sludge with shiny black surface deposits on surfaces in your engine. Many (most) corrosion inhibitors in oils and fuels are nitrose-amines (think Shell Nitrogen Enriched! Gasoline), which adhere to metal, forming blackish protective films. But very thin and hopefully uniform.
If it were me, I'd do what you did, first try to determine what has changed, about my air filter, riding habits, type of dirt typically encountered, etc. More difficult to determine, maybe the manufacturer has changed the formulation of your oil... that happens all the time. The specs don't change, but the base stock and the additive package can change, and they typically don't call ya up and let ya know...
I'd change oil and filter more often for a couple of changes. Switch to a cheap oil, run it for a few hours, change it out hot hot hot - repeat. Try to flush the crap out.
And again, if it were me, I'd change my oil manufacturer, maybe even my filter manufacturer. Go for the big filters. Just to see. I'd switch to a different brand for a bit, see if anything changes. Maybe even a car oil for a couple thousand km, one designed for a higher-mileage vehicle - the additive packages in these types of oils have a very agressive detergent/dispersant pack.
These are simple to try, and I'm sure most would have thought of them. Anything more advanced would need a disassembly and a mechanic - and I'm not one.
Just my opinion. I do have some experience in the manufacturing side of oils and additives, but I don't know everything...