Use engine
oil on the cylinder walls,pistons and rings. You want to lube them at start up. Not a good idea to use assembly fluid in this area.
Toploaders are made for SAE 90 and it is a good idea to stick with what it was made for. The toploaders are a bit on the loose side not like the newer trans that are built tighter and can get away with the light weight oils.
Don't forget to get a new head gasket to replace the one you did the mock up with.