I see no reason why I cannot use automotive coolant in my dirtbike radiators. But the guy at the bike shop says I need to buy this expensive stuff he sells. He also says I should use his $8 a quart tranny oil instead of the 10W 40 I am using. Can I just use some prestone anti-freeze and mineral free water? It's getting cold here in OK. Should I still use a 50/50 mix? Matt