The T5 can be run on the 75W90 gear oil, but it will kill the synchro's over time.
The best oil that is the closest to the oil designed/chosen for it is the CASTROL TQ-D, a ATF but a certain variety, available at Halfords, about £9 a litre, you'll need 2.55 litres from empty.
The engine oil really varies a lot, if the engine is a bit old/worn I'd be using a 50 weight or even a 60 weight, 2 that come to mind are Fuchs Titan 5W50 (silkolene pro s), and Millers 10W60.
Ford specified a 5W30, which would spell death to a cossie, if used as most do.
A new/unworn engine would be ok on a 40 weight if driven easily, a oil that is thicker to start with will still be thicker at the same temp as a thinner starting one, hence more protection from surface to surface contact.
tabetha