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AKI is the average of RON and MON. You can usually assume AKI to be around 4 points lower than RON (although this is not strictly true, it's close enough) so he should be safe with 87, if the car is healthy. 89 is cheap insurance if the car has some miles on it and may not be in perfect tune. The hot running is probably the radiator. If it hasn't had a steady diet of coolant changes using BMW's coolant, or the equivalent, I'd say just put a radiator in it. You didn't say what year his car was made. Some genertions of BMW's use a lot of plastic in their radiators and most of us BMW nuts consider those to be past their useful life after 60k miles. Failures at the upper hose neck are common.
JR
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