Additive in water for Alumi heads?

I like using over the counter premixed stuff from my local Honda dealership. It's an OAT formula made for all-aluminum or nearly all-aluminum applications and it also has a surfactant included (which is what water wetter is.)

Using some RMI-25 is also a good bet; it's a mildly "soapy" cleaner, lubricant, and surfactant all in one. However, because of its cleaning properties it can "find" leaks though in older or poorly maintained cooling systems once it cleans the buildup off.

You can also run magnesium sacrificial anodes in both heads and in the block plugs. You can get them from McMaster-carr. They're made for water heater use, but just cut them to length (keep them as long as possible) and thread them in.

Basically, the anode (zinc or magnesium) sacrifices itself so your aluminum doesn't have to.