i use 5-30w oil only as its fully sinthetic and apparently better for the trubo.. as the fully synthetic doesnt coagulate in the turbo when the engine is turned off, whereas the semi (10-40w ) can..
when you say flexi has gone what do you mean.....is it blowing?? as its common for the trumpet ends either side of the flexi to come away from the pipes ..and this isnt a problem..(sons astra gsi just past MOT like this)
Re oil leak , just give everywhere a good wipe over and place news paper under car engine and run till temp up to normal ...88/90deg and look to where the drips are coming from , then have a closer inspection..to find exactly ...then let us know on here and amybe we can advise further..
Also good to hear about your orig airbox being better than k&n..unit
Drives a Zafira GSI Turbo..in Arden Blue