Doing a mower oil change especially on a modern mower is a gift, the manufacturers are making them so diy user friendly, I doubt you'll need a tool.
So when should you do a mower oil change? Lawn mower engine oil should changed at least once per season or every 50 hours of operation. Most engines will take a !/2 quart (.6lt) of 10w30 engine oil.
If your yard is challenging terrain, hilly, over an acre, dusty, then a second oil change mid way through the season will help protect the motor. Clean the air filter regularly, about every 25 hours, more often in very dry dusty conditions.
