I took the time to call Royal Purple, and the guy was adamant that you do NOT add any more modifier to their gear oil; it already has it. I realize there is lots of experience on this forum, but I don't know how anyone here can just generalize and say all Royal Purple products are junk or that you always want to add additional modifier unless someone has sent the fluid away for analysis. After all, too much modifier can also be bad also, no?
I didn't add any to mine when I did the RP. My 4.10s always whined a little if I listened close enough, and exponentially more once I did multiple BMR suspension components with poly bushings -- to be expected. To my knowledge, I don't have an "chattering" that would occur if I needed more modifier.
I use Royal Purple 5W-30 HPS in my 2010 4.6 (with small Roush M90 blower). Between the synerlec and the elevated levels of ZDDP, I'm a happy camper. Runs smooth as a baby's bum.