Dont lean below 3000ft
And just how often have you heard that !.
Well I,v had it said to me thousands of times over the last twenty years , but I have yet to see it writen in a Lycoming or Continental manual.
The fact is that it is a bit of aviation folk lore left over from the dark past of british aviation when if it was a light aircraft the thing that powerd it was a Gypsy Major.
Yes guys its a Gypsy Major limitation ! ............for all other types read the flight manual and save your selfs 20% on the fuel bill.