Bringing this back for some advice on which fluid I should be using...

Finally got it all installed and with a Competition Clutch stage 4 and took for a test today. I know to expect some clunking etc from the diff but it's pretty severe, way more than I expected. I'm not just talking carpark speeds either, even a bit of clunking going around corners at 60kmh.

Driveshafts weren't changed and are pretty new and I have brand new 62a Hasport mounts. It only does it under load/acceleration so I'm pretty sure I can rule out anything other than the diff.

Currently running OEM MTF with a dash of the Ford friction modifier (40ml) which didn't seem to make any difference. Not sure how much to put in as it's really hard to get good answers from Google etc! The car is almost undriveable at low speeds so need some options here.