I'd never put conventional in a DI/turbo such as the Cruze. The first gens (port-injection, not DI) have acquired a generally poor reputation, due as much as anything to turbo failures as early as 20-30K - because GM initially spec'd Dexos1 gen 1 syn blend @ 10K intervals. I've always run my '13 with Synpower at 5K intervals and, gee, it's at 108K and has been trouble-free./quote]
The same is true for Saab's (especially the 9-5 and 9-3 from around the year 2000. Turbo's need high quality temperature stable oil that wont form deposits in the extreme conditions found inside the turbo bearing assembly.
I got my 9-5 at 150,000kms, it had a turbo failure (and replacement) at 120,000kms (~75,000miles) with a full service history and the recommended 12-month or 15,000kms oil changes with semi-syn as per Saab/GM specifications. Once I got the car I spent a long time 'de-sludging' the engine including pulling the sump and multiple oil flushes. Now only run Helix Ultra mid or low SAPS oil. The lifer tick went away, fuel economy improved significantly and it now drives like a brand new car with no smoke and smooth response. I'd never run mineral oils in my Saab, turbo race car or my Nissan GTi-R Pulsar 2.0LT. Turbo's need the cleanest of clean oils for a long and happy life.
While many fleet, off road and stationary large diesel engines run on Mineral oils (typically 15W40,) they:
-Are almost never shut down (and there is often a shutdown procedure that involves a 5-10 minute idle/turbo cooldown, to reduce the risk of deposits (coking) forming in the bearings. When was the last time you did a turbo cooldown on your personal car?)
-Run multi stage oil filtration systems, (these filters are often swapped out during an OCI with analysis done on the oil)
-Have massive sumps (dilution and temperature control)
-Operate at lower temperatures than petrol turbochargers (rare to see a diesel turbo glow bright red, even under heavy load, while it's easy to get a petrol turbo cherry red)
-Operate at a single sustained speed (significantly reduces thrust damage to the bearings cause by compressor surging during gear changes or on/off throttle applications.)
For these reasons the coking and oil grit that typically is associated with mineral oils in passenger turbo applications largely do-not arise. In severe non-watercooled petrol turbo applications where the bearing housing temperatures can easily exceed 4-500°C Poorer mineral oils can simply breakdown and varnish in the bearing during operation. Remember the residency time in the bearing for the oil is short. But the thermal stability of synthetics is of significant benefit here. GTL and PAO base oils providing the best stability. (easy to see now why I use Helix ultra in my vehicles
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These reasons are also why sustained highway driving (even with a heavy load) puts the least wear on your lubrication system. Everything is at steady state, with sufficient airflow to keep all temperatures in check. Oil pressure is up and since the engine is under load, cylinder temps+pressures are up, limiting build-ups and in some cases actually burning them off (the good old Italian tune-up.)