Subject truck is a 2006 Freightliner M2 with Cat C7 engine, 11293 kms on the oil. Engine has a deep sump, holding 28 l of oil. Last fill was in December 2016. The viscosity is down to 12.9. On another truck ( 1999 Freightliner FL70 with Cummins ISB 5.9, 6500 kms on oil ) with the same Delvac ESP 0W40 fill, the viscosity is 14.4. I usually change the oil out at 8000 kms. This time I let it go longer, mostly because I expected the synthetic Delvac to last longer. Is this 12.9 vs 14.4 significant? Should I not exceed the 8000 km change interval? Should the synthetic oil have not sheared down as quickly?