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Oil is not oil. Synthetic oils are "engineered and manufactured" - ok buzzwords I know, but basically they are synthesized from basic smaller molecules to yield an almost pure base stock. These base stocks are typically polyolefins or diester oils. Amsoil uses both. These base stocks are then blended with the best antiwear additives, neutralizing agents, defoamers, detergents and dispersants (all in small amounts relative to the amount of base oil). Anyhow this oil package can withstand temperatures that destroy petroleum oil in a heartbeat.
Why? Refined petroleum oil is still a many component, nature made blend. All sorts of long and short chain oils. Some stable some not. Some volatile some not. When hot the volatile stuff boils off and the unstable molecules break. To make things worse along come the wide spread multi weight oils. 10W-40 and 20W-50....bad news for petroluem oils because they can't make this viscosity spread on their own. These oils need pour point agents to make them flow at low temps and viscosity enhancers to make them more viscous at high temps....the problem with these agents (solvents and thickeners) is that they are not stable either.
So what does this have to do with your turbo? Well the previous owner of my car always ran Castrol 20W-50. In fact he bragged about it. I wouldn't even use this oil in a putt-putt non turbo. That crap killed his turbos and he didn't even know it.
Jonathan is right, there was not much for consumers when it came to synthetic oil. Volvo didn't start recommending synthetic until the 1990's. (It's in my 1996 850 manual for example)
You must have a 1984 manual. In my 1985 manual on page 36 there is a "Turbo Caution" section: "Do not race engine when cold"....."let it idle a short time..to allow the compressor vanes to slow"...."After hard driving idle for a few minutes.....risk of heat damage and/or turbine seizure due to lack of lubrication".
Why? Because the turbine rides on an "oil bearing" - if the thing is still spinning at 100,000 rpm when the engine/oil pump/oil pressure stops the turbine bearing surfaces will litterally have nothing but metal to metal contact....WHOOPS!!!!!!!!!
I'm not hypeing the synthetic oil. You want your turbo to last. It's no joke.
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