Sticky gearbox - ATF?

[King Fisher]

Nick's noticed that his gearbox is a bit 'sticky' on colder mornings, and spoke to one of his tutors who worked at a Ford garage for over 20 years. What he said is that the Type 9 was prone to getting gunked up by normal 75W90 gearbox oil, and that Ford sent out a form of ATF to eliminate the problem. He said they only put a tiny amount into the gearbox, but that this cured the problem completely? Is this true, and if so, which particular ATF should Nick use? Thanks.