So long as the anti freeze says it is suitable for alloy engines will be fine, but shouldn't need to keep topping up, it has a leak, unless you are topping only to "low", some mk1 mondy like the level to be and indeed always settle at about 1/2" under the min mark but don't move from there, that's fine, my old mondy was like that keep topping to min and it spat it out to 1/2" under the min mark but never used any.
tabetha