That's some hard core way way old school tech. The motor's clearances are not designed for that AT ALL, and it won't work out well.
Here's a basic primer on motor oil weights - 5W20 means that when cold, the oil flows like 5 weight oil. This is important, as it allows the oil to flow to critical parts of the engine faster. 20 means it flows like 20 weight when warm. This is also important as it offers a slightly better barrier of protection. When hot, it will flow about the same as it did when it was cold.
Straight 30 will flow like 30 when it's cold, and 30 when it's hot - meaning it can't get to the tight clearances of modern engines with any urgency, leading to more start up wear/tear and no benefit.
No offense to the guy, he may be knowledgeable, but not about this.
we use atf because its cheaper for ford to just use one oil for everything. I have read that some motor oils are better for a transmission than the atf thrown in there. There are additives in motor oil that are better for a manual transmission than atf. I think one of the oils to use is high milage mobile one, maybe start googling that. Good luck.
Nonsense--Tremec recommends ATF for both the T-45 and T3650--and they have no horse in Ford's race. ATF is engineered to be the best for ball bearings and clutches (which synchro rings are), motor oil is not...