A small sight glass in the crankcase can show the engine oil level, but reading it involves more than looking at the window. In one Yamaha owner’s manual, the engine is warmed first, then stopped long enough for the oil to settle. The motorcycle is held upright on level ground for the reading. If it leans on a side stand, the visible line may not represent the true level. The conditions of the check matter as much as the line you see through the glass.
The sight glass described in that manual has minimum and maximum marks, with the intended level between them. For this particular motorcycle, Yamaha says to add the recommended oil if the level is at or below the lower mark. The filler cap must be secured afterward, and any leak deserves attention. Other motorcycles may use a dipstick, put the window elsewhere, or call for a different sequence. A single warm-up time or top-up quantity should therefore not be treated as universal advice.
The useful habit is to read the oil-check section of your own motorcycle’s manual before you need it, rather than relying on a procedure remembered from another model. That same manual specifies the oil type and service interval. If the level drops unexpectedly, there is a visible leak, or the reading remains unclear, ask a qualified workshop to inspect it. A meaningful reading begins with a stable motorcycle and the maker’s procedure; the sight glass supplies the answer only after those conditions are met.