I am in the UK and have a 98 318iSE Touring with 11000 miles and presently use Castrol GTX Advanced Magnatec 10-40 which is semi - synthetic.
I think it would be advisable to switch to Mobil 1 0-40 can anyone advise me if this would be advantageous. The car is really a second car, lightly driven and oil changed every 3 to 5 thousand miles and dealer serviced every year. Also would it help to use synthetic oil in the gearbox and diff.
I intend to keep the car for a very long time.
I would apreciate your advice, thank you.
Roger

If like you said you intend to keep the car for a long time then full synthetic is the way to go.

I live in Germany and drive a 95 318i. I use Valvoline's 10-40 and in winter the 0-40( synthetic)