morning all, just bought a 98 1.8 se petrol a4 in silver mint condition drove around 50 miles in it so far with no problems seems a bargain at 700 notes. just a quick question is semi synthetic 10-40 oil ok in this vehicle it has done 158,000. thank you.

I'd stick with full-on synthetic, given the sludge issues these engines are prone to.

use mobile one or castrol gtx fully synthetic

Castrol GTX is good for what it is but it's not synthetic.

Use Mobil 1 0w40 (NOT 5w30)

Its really preference mostly. I WOULD go FULL synthetic but brand is on you. I run Castrol full synthetic (black bottle white green label) 5w40 winter (cold) and 10w40 summer (warm). I find most owners of a foreign car either use that or Mobile 1 Full synthetic.

Is that a non- turbo? Don't see to many of them around. Honestly, I would check the owners manual on this one...

good catch, +1 on that

Nah, same oil is good. There might be more oils that are okay for it but all the stuff that's good for 1.8T is good for 1.8.