To change the gear oil on the G86 or G86/20 will require a special tool. The tool is a tamper proof 19mm 12 point (Hazet 2567-16). The gear oil is to be filled 11mm from the side fill hole on the G86 (2.7 or 2.5 Boxster). To achieve this put in 2.25l of fluid and check. This hole is on the drivers side just forward of the output shaft and the drain is on the bottom. The G86/20 should be filled with 2.8l or until it just starts to come out of the hole. When you reinstall these torque them to 19ftlb. The proper gear oil Porsche recommends is SAE 75 W 90 Burmah - Oil TAF 21.
You will have to jack up the rear of the vehicle and remove the rear stress plate under the transmission. The drain is right under the transmission between the axles and the fill is to the rear of the drivers axle. You will need a special tool to drain the oil. It is a Hazet tool number 2567-16. It is about 40.00 and you can order it thru http://www.themotoringshop.com/