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I’ve given up for today, it took me quite some to remove the Final Drive/IRS oil drain and filler plugs out of the 420 and get the old oil out.
Now I’ve the challenge of re-filling the unit with new oil.
Short of filling my oil can oil and squirting it in a bit at a time, has anyone got any sensible suggestions on how I can get the new oil back in please?
I thought about connecting a small flexible hose to top of the new gear oil plastic bottle dispenser and just squeezing it but can see that might get a bit messy.
Just like water, oil won’t run uphill...
Cheers, Mac.

A small hand-operated mini pump like the one below will make short work of it. Costs around £15 online. This one is a Sealey TP6804 1 litre mini pump.
I have a garden pressure sprayer that I've adapted which is fine for things like transmission fluid that flows better but differential oil is much thicker and so it takes longer.
The little pump below pumps directly so will probably be easier for diff oil. You will only need to refill the bottle once as I think the diff takes just over 1.5 litres.
Hope that helps!

Ah! .... thanks David.... now why didn't I think of that!