Not everyone realizes that motorcycles and other powersports vehicles did not come under emissions regulations until over 20 years after cars did. For motorcycles this was during the 1978 model year. Motorcycles did however get some emissions-anticipating specifications a little bit prior to this, in fact as early as the mid 1970s, one of which was crankcase recirculation. Crankcase recirculation has no formal name in powersports but is known in the car world as positive crankcase ventilation or PCV. An ancient and abiding specification on cars, powersports vehicle crankcase fume reingestion from the crankcase to the air intake in most cases has even today pretty benign effects on performance and maintenance while it of course improves hydrocarbon emissions. Again, just as on cars. No worries. Generally speaking.
But there are a few problems with this system on certain motorcycles. For one, on all powersports machines having this crankcase rebreathing, it means that over-filling of the crankcase (whether unintentionally through incorrect oil change technique, or due to either fuel or water dilution of the engine oil artificially raising its level) can affect the engine's intake system in some not-so-good ways, there being a more or less straight path from crankcase to intake. Even properly maintained crankcases on certain vehicles whose crankcase breathers are perhaps mounted a bit to close to their throttle bodies, can exhibit symptoms of crankcase pollution of their fuel systems. The result is fouled air cleaners, the accumulation of grime inside throttle bodies and carburetors, and accelerated carbon formation on intake valves. Not so good.
CV-carbureted bikes such as the venerable and highly valued late 70s/early 80s Honda multis -- of which I am an ardent, active supporter -- are especially vulnerable. Distilled crankcase hydrocarbons form a greasy slime on these carbs' moving parts, inhibiting proper function and when built up around the throttle butterflies, can affect proper engine idle. Hondas and Kawasakis using metal piston CVs form this black grease most notably on their slides, a sign that the crankcase is having undue influence on these machines' intake systems, and ultimately requiring cleaning of these parts to recover lost performance.
At the very least it is imperative on metal piston CV models that the airbox "dump" hose be inspected periodically (squeezed) to eliminate the buildup of distilled crankcase vapors. Consequently, knowing that most folks like me are lazy, I routinely encourage altogether defeating the crankcase recirculation plumbing on bikes having metal piston CVs. It's easy to do, reaps maintenance dividends and, well, I simply hate like hell seeing grunge being fed into carburetors!
Alright. One more thing. Now, knowing this, that these bikes are already predisposed to foul their intakes with the accumulation of hydrocarbon's black goo, you may want to think twice about introducing additives to either the fuel or oil systems.