Oxide.Core.OxideMod.Version.Major.ToString("0") + "." + Oxide.Core.OxideMod.Version.Minor.ToString("0") + "." + Oxide.Core.OxideMod.Version.Patch.ToString("0");
Always returns 2.0.3867 ...should be 2.0.4244 ?

Each extension has its own version. 2.0.3867 is the version for Oxide.Core. You can get the Rust extension version from RustExtension.Version.

Also, you don't need to split it, just use OxideMod.Version.ToString() or string interpolation.

var ov = Oxide.Game.Rust.RustExtension.Version;
compile error an object reference is required to access non static member

Should be RustExtension.Version, that's what we use in the extension.

doesnt compile, what directive (using) should it be under?

Merged post

var ov = Oxide.Game.Rust.RustExtension.VersionNumber;

does not contain a definition for VersionNumber