Son and I will be doing the timing belt and water pump on his 09 OB 2.5i this weekend. Odo is at 210K but this engine was put in ~125K with unknown mileage on it. Engine leaks only a very small amount of oil and seems pretty solid for the most part. When changing coolant what I've done on other past vehicles (non Subaru) is use something like Prestone coolant flush/cleaner and drive ~40 miles or so before fully draining and flushing the old coolant out. What I am concerned about though is if this cleaner might cause head gasket issues or leaks. Thoughts on using the cleaner or just flushing it really good with distilled water only?