Gerry Dalton

During sailing with the motor off the gear lever was accidently engaged to forward. After an hour of sailing the motor was started and forward gear was engaged which resulted in a great deal of slipping from the gearbox. The gearbox oil level was found low and topped up which reduced the slipping however it still slips at high revs. Is this the result of sailing in gear or is it a coincidence? Will adjusting the gear linkage assist in resolving this problem?