Mark1234
You must be flying Tiger Moths as that is the only place that I have seen a limitation on leaning the mixture below 5000ft!
But I suspect Not.................
I Think that the people that you are flying with are going to have a big problem by publishing rules that are in conflict with the flight manual, lets take a look at what might happen....................
A newish PPL who trained at this club takes an aircraft on a a long trip and flys the aircraft IAW the club rules on not leaning the mixture, Unfortunaly he runs out of fuel and in the resulting ditching someone dies.
Six months later in court the lawyer for the dependants of the victim gets hold of the fact that the club rule on leaning the mixture reduces the range of the aircraft by IRO 20%. So we now have the court looking at a club rule that resulted in an aircraft running out of fuel when if the aircraft had been flown IAW the flight manual a safe landing could have been made.
If you are running a business you are a fool if you issue rules that contermand the flight manual, it can only end one way in court.