What causes it
To burn off soot trapped in the DPF, the engine periodically injects extra fuel late in the combustion cycle to raise exhaust temperatures — a process called regeneration. On engines used mostly for short journeys, regeneration often can't complete properly, and some of that unburned fuel finds its way past the piston rings and into the sump, where it mixes with and thins the engine oil.
Why diluted oil causes problems
- Thinner oil provides less protection at the timing chain tensioner, accelerating wear and rattle
- Reduced lubrication at the turbocharger bearings, increasing the risk of premature turbo failure
- Oil level can rise noticeably above the maximum mark — a useful early warning sign
What we check for
Whenever we're working on an Ingenium diesel — particularly during timing chain or turbocharger repairs — we check for signs of oil dilution as standard, since it's so often a contributing factor. If your vehicle is used mainly for short journeys, we'd also recommend more frequent oil changes than the standard interval to help manage the risk.