My wifes Cayenne service due indicator just lit up on dash.

Car has only 18000 miles on it

We did 1st service at dealer when indicator lit at 13000 miles & they
were OK - it was approx $300.

I called to check cost on this service & they quoted $560 - when I
asked what they wil be doing they said oil change & then check this,
this & this, etc

I called 2 other local dealers & there prices were $6-800

I called a local recommended Porsche mechanic with a shop in my town &
he said it is just an oil chane, approx $100 & he will inspect
everything & let me know if any other work is needed.

Car has 2.5 more years on 5 year lease.

Is it a warranty or lease issue if we do not service at dealer?

Any advice?