Hi,
there are hundreds of thousands of turbo cars and trucks that have never had a synthetic oil near them. They have survived. Early turbo's during the 60's & 70's were prone to premature failure due to wrong placement causing rapid oil drain back - and next to no idle down, along with early oil technology
Very few car makers had a diesel oil class recommended back then ( eg. SJ/CD Cat Series 3, etc )except from memory Mercedes, VW and Porsche. They all probably should have!
Yes, Porsche approval is a good point but so is the MB <229.0> approvals along with A3-96>/B3-96>, and a mandatory diesel rating of CD+
Turbo's are best treated as a mandatory idle-down deveice and usually one minute is enough in most cases. This also stabilises the coolant temperature reducing localised "hot spots" etc.
In the 60's I replaced many turbos in earthmovers (dozers, loaders, scrapers etc. ) and in heavy trucks due to "wear out" - they do not generally give problems in installations today. And especially not in cars
I must say though that I would only run synthetics today in my gear - in a wide range of motorbikes, cars, utes and trucks
Subaru calls for very light oils in their turbo cars today - to help prevent turbo lag on accelleration, along with other reasons
Doug Hillary
Airlie Beach - Tropical North Queensland - Australia
MY02 Subaru Outback 2.5
MY98 BMW Z3 2.8
MY89 Porsche 928 S4