You had me at endless free supply.
Most light duty diesel cars take a ACEA C3 oil with Sulphated Ash < 0.8 %
The heavy duty diesel E7 has a SA < 2.0 %
Closer would be E9 with SA < 1.0 %
But they all (C3, E7, E9) have a HTHS > 3.5 cP.
My only concern with the high SAPS (high SA) oil is your DPF exhaust filter. No DPF then no worries.
Penrite Vantage 10W40 SN & A3/B4 + Wesfil-Cooper Z154