What type of transmission fluid?

The car drives really good and the transmission feels great and shifts nicely but will not stay in 5th gear without keeping constant forward pressure on the shifter. It will pop back to neutral almost instantly when you let go of the shifter. I was going to do a drain and fill of the fluid and see if that helped. Just wanted to know what type of fluid and how much the transmission holds.
Is this a common issue and any suggestions for a fix would be great!
Thank you,
KiKi