I have a 1997 Ford Ranger with 420,000 miles on it. I drive this thing a lot and have been changing the coolant every 30,000 miles for some time. The WalMart Supertech ethylene glycol coolant I use says it is good for 150,000 miles. I am thinking about increasing my coolant change interval to 100,000 miles which coincides with the frequency I change the timing belt and water pump. I know that coolant technology has improved significantly since 1997 when Ford recommended a 30,000 mile interval but I don’t know what I don’t know about coolant. Interested in thoughts, especially from anyone who has done something similar in an older vehicle.