MG Car Repair: 74 MGB OD slipping - follow up, correct oil, gear oil

Question
Thanks Howard on your answer to '74 MGB OD slipping. I will do the suggested troubleshooting. You recently recommended using 20W50 oil for the gear box. I recently changed it but used an 80\90 gear oil. Is it harmful, and should I re-fill with correct oil? ( It seems to be shifting OK, although the underlying dropping out of gear still happens )

Answer
Hi Paul,
Even though BLM recommends gear oil we got a lot better performance from using engine oil in the gearboxes with and without Overdrives. What do you mean "dropping out of gear" Do you mean you have to put the shift lever back into a gear? Or it starts slipping?

Howard