Oil capacity? what is the engine oil capacity of a 1985 yamaha maxim XJ 700?

Should be 3.5 Liters / quarts with the filter change. Put in 3 liters then top up as needed to get the level to the top of the viewing window.

To check your oil, bike should be level, on it's center stand, engine should NOT be running!

Any oil viscosity between 10w30 and 20w50 will work just fine. Smaller numbers if the weather's cooler and/or you only do short rides. Bigger numbers for hot weather and longer rides.