I wouldn't worry too much about some smoke coming out of the oil filler cap. That's blowby past the piston rings, and all engines with some mileage on them have a bit of blowby. The blowby is normally handled while the engine is running by the Positive Crankcase Ventilation ( PCV) system, which routes the crankcase fumes ( blowby) back into the engine intake , where it is burned along with the fuel/air mixture. It's important, however, to make sure your PCV system is clear, and working properly. There's a PCV valve that regulates the flow of blowby , usually from one rocker cover ( valve cover) through a hose, and into the intake through a port in the base of the carburetor, fuel injection base, or directly into the intake manifold. There is also a second part of the PCV system which admits fresh air from under the air filter into the other rocker cover. PCV valves are cheap to buy. However, if the engine has a lot of blowby, the intake port at the base of the carb. or fuel injection body can be clogged solid with hard black deposits , which must be removed before the PCV system can operate.Disconnect the PCV valve from the hose leading to the intake, and if you can blow air through the hose into the intake, then it's not clogged. As for piston rings, if your engine is not burning much oil, not fouling the spark plugs with black deposits, and accelerates well without spewing a lot of smoke from the exhaust, then you have nothing to worry about. You won't damage the engine even if you're burning a little oil, such as less than a quart every 1500 miles. (And note that many engines leak more oil onto the ground than they burn.) A compression or leakdown test on each cylinder can give you more information on the condition of your compression rings and valves and valve seats. If your piston rings really need replacing, it can be a big job. To do it properly, the cylinder bores may have to be bored or precision honed, which means removing the entire engine.
