Only for advanced users. Flashing the wrong preloader can hard-brick the device.
Yes. Oppo F1s never received Android 8.0 Oreo or later. The final official version is Android 7.1.2 with security patches up to December 2018. If you need newer Android features, consider custom ROMs like LineageOS 14.1 (still Android 7.1) or unofficial LineageOS 15.1 (Oreo), but note they require an unlocked bootloader and have bugs. Oppo F1s Firmware Download 7-1-2
To flash Android 7.1.2, you need TWRP (Team Win Recovery Project) for Oppo F1s (A1601). Only for advanced users