Only to add there is so much conflicting advice about whether to lean or not.
Potentially there is concensus that it is worth leaning at higher levels.
At lower levels opinions seem more mixed.
Whatever view you hold the danger is leaning aggressively without the benefit of EGT and CHT probes on each of the cylinders. The consequence can be to leave one or more cylinders running hot (because you have no idea it/they are running hot). The end result will be cylinder damage or worse.
I can give you a good example where with my own aircraft one of the cylinders was running significantly hotter than the others when the engine was "fully" leaned and yet from sound and feel the engine showed no indication what so ever of a problem. The plug was beginning to fail.
Given the cost of an overhaul against the fuel saved by leaning I cant see the sense in "fully" leaning an engine unless you have some means of monitoring cylinders temps. I would run richer, take comfort in the extra cooling and pleasure that my engine will get much closer to TBO. If it were my own aircraft one of the first things I would do is add a temp gauge - they are relatively cheap, easy to install and worth their weight in gold.
Sadly of course when it comes to rental aircraft many pilots are less worried about the engine (which isnt theirs) or the next pilot that is going to be flying than their wallets, although at least when the aircraft is rented on wet tac time this is likely to be less of an issue.