Topping up refer compressor oil by two methods

Refer Compressor Oil Changing 

• Close the condenser outlet valve
• Collect the refrigerant into the condenser. stop the compression and shut the inlet and outlet valve of the Compressor.
• Connect the flexible changing have of the piston pump to the oil filling plug.
• Purge the air and tighten the connection.
• Start filling the air slowly, after filling to the correct level, stop filling and disconnect the hose.
• Make the system normal.

Tapping up can be done by two procedures:

1. With a separate oil pump

• Without affecting the operation of the compressor, the oil may be topped up by means of a separate oil pump.
• This pump enables all to be forced into the crankcase via the oil charging valve against suction pressure.

2. With the crankcase pressure below atmospheric pressure

• Connect a hose to the air charging valve after removing the cap nut with the gasket.
• Fill the hose with oil & emerge it's loose and in a reservoir containing a sufficient amount of oil.
• Close the suction stop valve to such an extent that the suction pressure falls below the atmospheric pressure.
• Keep the oil charge valve open as long as the oil in the crankcase has attained the required level again. Take care that no air is being drained into the crankcase.
• Slowly open the suction stop valve in order to prevent liquid hammering.
• Remove the host and replace the cap nut with a gasket onto the oil charging valve.

(Note: compressor takes suction via crankcase)