Here's the deal.....
I read in my Clymers Mercruiser manual that the Oildyne/Singer pump (plastic reservoir) and the Oildyne/Eaton (metal canister) are interchangeable. Also, it states the Singer (Plastic tank) should be filled with SAE 10w-30 or 20w-40 engine oil...... My Merc dealer says to use the Merc Fluid only in both! AND to make matters worse, the metal canister pump label says to use ATF only !!!!

What is the right/correct thing to do here? !!!

I would really like to bleed the hydraulics this weekend, so I need to know asap!!
Thanks in advance.

Any of the choices you mentioned. As long as you use what's already in the reservoir. Don't mix fluid types.
When I had to flush my trim system I refilled it with 10-30 Mobil1.

I filled mine with ATF.

I used straight 30w. It's a hydraulic pump. If you could control the corrosion water would even work. Don't sweat it.

Excellent!! The pumps & reservoirs were disassembled, cleaned & new seals installed... so they are "virgins". The fluid in them was very, very thick & milky with junk in the bottom of the reservoirs....... I think I'll go with the 10W solution!!

ATF in my trim pumps and tab pumps in the Formula (from the factory).

ATF in the Bennett pumps in the Sea Ray (from the factory).