i have a manual transmission e34 520i 24v on a m50 non-vanos engine.the car is now touching 221,000 km ~ 131,000 miles.questions then:1. for this particular m50 non-vanos head, what are the frequent problems/fixes ?2. what coolant besides bmw blue coolant would be best (phosphate free etc) - would regular shell/esso coolant do ? what about distilled water?3. any other issues with related this particular engine head ?