I recently bought a 98 V70 XC with burned exhaust valve. While removing the head for valve job, I noticed a coating of oil in the air hoses and intake manifold. Is this normal?

 

Some oil is normal....

Check the turbo unit. spin the shaft and wiggle it.....see if it has any serious play. It should not touch the compressor walls....etc.
It should spin freely.

Might want to replace all the gaskets and PCV system while you in there????

Previous owner was probably running way to low of Octane...91+ for turbos.