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Syntec is a Group III severely hydrotreated mineral oil. This process (which is not a "full" synthetic) uses high pressures and temperatures to alter the polymer chains, producing oil base stocks with high viscosity indices that resemble Group IV and V PAOs, esters, and PAGs, but with higher pour points. These higher pour points are reduced down to acceptable "synthetic" levels with pour point depressants. Castrol, Pennzoil, Quaker State, and a few others use this technique, which is less costly than producing a full synthetic. The oil performs very well, but not as well as a full synthetic such as one from Mobil or Valvoline.
The viscosity used should be on the low side to improve cold pumping and fuel economy. Maintain good lubricity by selecting a first class base stock and additive package with the minimum viscosity index improvers (which do no actual lubricating) to achieve the viscosity range you need. In all likelihood, Syntec 5-50 and all others in this range are laced with VI improvers, which break down in use.
Volvo recommends 10-30 for almost all use, or 5-30 if you use a synthetic. If you are going to haul a trailer in the mountains or the like, I would absolutely use a 10-30 full synthetic. I did try Syntec 5-50 when I first bought my Volvo wagon and it resulted in a front engine seal that was sucked inwards due to (I suspect) increased engine vacuum because of lower blowby.
I now use Mobil 1 0-30 in the Iowa winters and 5-30 in the summer for my NA engines and 10-30 for my turbos.
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