5w-30 Syntec should pump acceptably down to ~-25C.
Check the sender or just replace it. It's ~$25 depending on your dealer. You might catch me whining about the recent warranty expiration to improve price.
The Syntec we get in the USA is a group III (semi-synth) but unless you've really abused the change intervals (10K multiple times) you're doing fine. 4K miles is just getting warmed up.
You're maybe at 1/4 of the engine's life. Those I6's can last a very long time.