Keeping a heater core in good shape REQUIRES the use of enough anti-corrosion additives for protection. All "decent" antifreezes contain these, but the cheap green stuff's silicates don't last too long, ~2 years tops.
Run it at 50% and change it every 2 years and that's about all you can do. I'd avoid the orange stuff, it usually works but there's too many horror stories for me.
For my "good" engines I use the $26/gal Toyota red antifreeze (has lots of corrosion-fighting phosphates that last many years--no silicates), but you MUST use only distilled/very soft water with it.
US out of the UN, UN out of the US.