[Arragonis]

If the Yaris has VVT then it may be an oil pressure issue, I think (maybe wrongly) the VVT system uses oil pressure to make its adjustments. Maybe a non-return filter may help.

dremd - you may be right. A lot of VRS owners have put down turbo failure to not allowing the turbo to cool - kind of you hammer down the motorway at 70-90, stop for a pee and shut the engine off in the rest area / services straight away. Do it enough times and the oil in the turbo stays there happily cooking.

Not sure about coking, the issue reported is that the seals start to fail rather allowing oil into the turbo and lots of blue smoke, rather than the bearings failing which would be poor oil supply due to it coking.

Of course I could be wrong too. I feed mine the VW spec 505.x (I think thats the spec) synthetic oil and at the moment it uses non between changes which is why my I looked elsewhere for my recent turbo smoke than worn parts. Luckily my gaffer tape / turbo boost pipe fix seems to have cured it.