Snowy Astra - you're actualy destroing your turbocharger apart form averything else by usin non-synthethic oils. I already asked you once on similar occasion - did you use proper synthethic targeted at performance/sports cars? In motorsport nowedays there are only synthethic oils even in old archaic constrution cars. On very oil demanding rallycross group A air-cooled Fiat 126p with which I used to help during events we used Castrol TWS 10W60 with no problem. Even though on this car cooling dificency is so big that it easily sees temperatures like 140-150 degrees and starts to "knock" at the end of each race.