Definitely makes a difference. What were you previously mixing at? And what oil are you wanting to run at 40:1?
Rich/lean terminology applies a couple different ways.
Air/Fuel Ratio: It's the ratio of air to fuel that creates a proper burn with out creating so much heat that it melts pistons, but also doesn't have to much fuel so that it create a sloppy cool burn. Think of when you start up a torch and play with the mix valve. That burn you get before you squeeze the lever is what we want in our cylinders. That burn you get when you squeeze the trigger is basically what you get when an air leak occurs.
Fuel/Oil Ratio: This applies to your question. If a given motor runs perfect with the current jets in the carbs, changing the amount of oil in the fuel can have an effect on jetting. So say the motor is running perfect. Now take a motor that's been running at 32:1 fuel. Now you add fuel mixed at 40:1. You are actually giving the motor more fuel through the carb jets. Not to big of a deal. Now say you have a motor running perfect at 40:1. Take that motor and feed it 32:1 mix. You would actually be flowing less gas through the jets. Typically the average motor is jetted with a decent margin of error so you may not notice the difference. Do not let people confuse you by calling it oil/fuel ratio when you read a number XX:Y , fuel is the first number in the ratio.