Hi Hugh,
Good advice would be for you to join the Yahoo Harrison User Group. There are loads of files you can access and download including oil charts. Plenty of other useful info as well.
You can also access and download a pdf copy of the manual, which has the lubrication charts.
Top gearbox is a thin ISO 68 hydraulic type oil. Bottom gearbox I am currently working on grade update comparisons but its an ISO 220.
Think some oil traders on Ebay actually list specific oilsfor the M300. As for Brands, you will never go far wrong if you can get Castrol in economic quantities. The machine capacities are much less than you would think.
As for gearbox rattles i would say that they are never really welcome. There is a few Youtube videos where there have been similar issues which at the end of the day have just been put down to wear and tear with no real cost effective solution. Its an easy job to lift the gearbox lid and check for visible damage, but avoid running with it off as oil will fling everywhere. I would just make sure you are correctltly lubed up if nothing visible is obvious.
Malc
Edited By MalcB on 02/01/2017 19:43:37
Edited By MalcB on 02/01/2017 19:51:08