i use the -100 allways , my reasoning is that there is water in the block still and the anti freeze is being diluted, when your running it though. in past ive bought just rv anti freeze hammer posted a while back to use a different anti freeze still safe for enviormennt but has lubricant in it so block and what ever else wont rust out.so iam going to look for that in west marine book. i run 5 gallons though one engine when i do mine better safe than sorry dont want anything cracking on me over winter. for water system i just use -50 make sure thermostat is open so itll circulate all through the engine. that works for me others may have better ideas.