My SO's SOC (L Reg Golf GTi) has some very specific instructions re coolant and anti-freeze mixes (preferably special VW number blah or to industry standard B$$GF£D172XC93... blah blah). And the coolant levels need topping up. Should I care about mixing any old (suitably diluted) antifreeze from the nearest petrol-station with what's already in there or do all these numbers actually matter?