I inherited a brushcutter when I bought a house and need your advice. It is a Tanaka Brush cutter TBC 321 The manual states that the gearset should be lubricated/serviced every 50 Hours. Does this mean "Check the oil" only. If so, how do you know when there is enough oil in it, could you over-fill it and most important which grade of oil should you use? SAE 20 30 ...SAE 90 ??? Please help! Thanks, Marius