I am flying from Sydney, Australia to Istanbul Turkey with a 20 hour layover at Peking Airport

You don’t need a visa because of 24 hours transit free policy and if you want to leave the airport you can fill in a table to apply for it before you arrive at the airport.

Thank you, when I look at the 24 hour visa free direct transit for passengers with Turkish passport it says a visa is need... Which visa would I need to apply for?

"Who Are Eligible for 24-Hour TWOV?
National of most countries are eligible for the 24-hour direct transit, while a visa is needed for the passengers transiting on Afghanistan, Azerbaijan, Iran, Iraq, Kazakhstan, Kyrgyzstan, Malaysia, Nigeria, Oman, Pakistan, Saudi Arabia, Sri Lanka, Thailand, Turkey, United Arab Emirates and Yemen passports"

I’ m sorry I got it wrong. You can apply for G visa which is for the people who will transit in China.