By default, OpenSUSE 13.1 already equipped with VirtualBox Guest Additions. Based on my own experiences, I can get the full screen, seamless mode on my OpenSUSE 13.1 when installed on VirtualBox 4.3.10. Then I update my VirtualBox to version 4.3.12. After that, I got a notification on OpenSUSE that Guest Additions 4.3.12 update is available.
This tutorial is going to show you How to update VirtualBox Guest Additions on OpenSUSE 13.1. First of all, we need to update the kernel in OpenSUSE 13.1. This single command should do.
zypper update kernel*
zypper in kernel-default-devel kernel-devel gcc
OpenSUSE 13.1 will reboot. After reboot, we can then install the new Guest Additions. Go to Devices | Install Guest Additions CD Image from the Virtualbox window. The installer should be automatically run and ask you the root password. If not, open Files and click VBOXGUESTADDITIONS_4.3.12_93733 (The version may vary). Then click Run Software
You will be asked to enter root password. Wait until the installation process is finished. Log out and log in back to activate the new Guest Additions.