In Biodiesel Fuel Research Working Group(WG) of Japan Auto-Oil Program(JATOP), some impacts of high biodiesel blends have been investigated from the viewpoints of fuel properties, stability, emissions, exhaust aftertreatment systems, cold driveability, mixing in engine oils, durability/reliability and so on. This report is designed to determine how high biodiesel blends affect oil quality through testing on 2005 regulations engines with DPFs. When blends of 10-20% rapeseed methyl ester (RME) with diesel fuel are employed with 10W-30 engine oil, the oil change interval is reduced to about a half due to a drop in oil pressure. The oil pressure drop occurs because of the reduced kinematic viscosity of engine oil, which resulting from dilution of poorly evaporated RME with engine oil and its accumulation, however, leading to increased wear of piston top rings and cylinder liners. When a blend of 10% hydrogenated biodiesel (HBD) with diesel fuel is employed with 10W-30 engine oil, the pressure drop is smaller than that of 10-20% RME blends, because HBD has less heavy fraction compared to RME, and thus, even when diluting with engine oil, HBD can evaporate and is not apt to accumulate. Another study has been conducted to determine how 10% RME blends affect oil quality. 15W-40 engine oil is employed due to high initial oil pressure to avoid oil changes during the test period, resulting from dilution of RME with engine oil and its accumulation. The oil changes are avoided as planned; however, engine oil shows indications of degradation, such as increased total acid number and lead elution from the bearing material and components.