I was amazed how dirty my diff oil had become at 9000 miles from new ( 3.5 years) when I changed it. In hindsight I'd have done it after the first year - I expect it'll stay cleaner after the initial change so its going to be a 2 year thing, like the brake fluid, for me.
The mazda gearbox by contrast had very clean oil at the same time/miles, so I think I'll be doing that every 5 years.
Here is a used oil analysis after 22,000 miles on the Mazda 6 speed box of the same vintage that Morgan use the 5 speed is a bit different to this.
http://www.billswebspace.com/11%20MIATA-TR-041412.jpgThe oil has dropped below the viscosity rating but not by a large amount.
Not that much of an issue at 22,000 miles as long as your mileage says similar to what you are doing at present as you will then hit around 22,000 miles.
If you have the facilites at home to change it. I would maybe do it sooner and use either Redline MT90 or Castrol Syntrans as that tends to give a slicker change in the Mazda boxes from changing the oil in a good number of them.
