Great info, thank you.Well, the key thing here is that you said your lube choices already have FM blended in. As such, there is generally not a reason to add more (thought some circumstances may require it). So, no, you don't need to add separate modifier. However, had you picked oil that was not blended with modifier, you'd have to add it generally speaking. Either way, a good-quality synthetic is important here. I know a lot of folks like their Red and Purple oils, though at least one axle manufacturer I've spoken with (whom I will not name) tells me that those products are "like using water". I've no test experience with either, but I will point out that the lube is critical to the ring & pinion gear, the bearings, and even the seals, rather than just the diff. In fact, the Torsen will run in most any reasonable oil. The R&P is, however, very fussy. I'd always default to what the manufacturer of the R&P suggests.
A side note with regards to the Torsen, Ford still uses modifier in that axle too. Helical gear diffs sometimes tend to "moan" rather than chatter, but FM fixes that. Ford wanted to avoid undue warranty claims from that, so they use modifier across the board. You could run it w/o FM, but it might lead to noise in tight, low-speed turns. But by using a pre-modified oil, you already have it, so its a non-issue.