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QI recently received a note from Mercedes-Benz saying that the engine warranty on my '99 ML320 is being extended to 10 years or 150,000 miles because the factory did not initially use or recommend synthetic oil as part of its Flexible Servicing schedule, which calls for oil and filter changes about every 10,000 miles. Apparently, when not using synthetic oil, these engine are susceptible to sludge and greatly reduced engine life. I regularly follow the MB service schedule, but when I opened the oil filler cap I was shocked to see a large amount of brownish "goo," with the consistency of whipped cream, in the cylinder head and on the cap. I showed this to the dealer, who said this was normal, and is "froth" from the oil reaching the cylinder head. The oil in my car is now the synthetic variety. I've never seen this before. The engine is running just fine. Is this normal? What should I do?

A What should you do? Be grateful for the extended warranty offered by Mercedes. The engine is now covered for a very large percentage of the vehicle's expected life.

Your responsibility from here on out is to use synthetic oil and change oil somewhere between 3,000 and 5,000 miles. Does this sound familiar? I've been recommending this "routine service" for years. Doing so will preempt formation of sludge and varnish from oil oxidation, preempt any viscosity breakdown of the lubricant (highly unlikely with synthetic oil anyway), and preempt depletion of the additive package before the oil is changed.

The oil sludge you found is more common than you'd think in modern engines. My family just purchased a used Volkswagen Passat in fine shape, but it had some evidence of sludge on the oil filler cap. Same reason: longer oil change intervals and the use of non-synthetic oil. We asked that the oil pan and valve cover be removed to inspect and clean the engine. This revealed minor sludge, which was thoroughly cleaned out. The dealer, Westside VW, switched the vehicle to synthetic oil, and recommended we change it 2,000 miles later to finish the internal cleaning process.

Given the vastly improved lubricants available today, why does sludge develop? Because of higher oil operating temperatures and significantly longer oil change intervals. You can't do anything about the higher oil operating temperatures -- which are not in any way harmful in their own right -- but you can do something about the longer oil change intervals.

Having identified the issue at roughly 50,000 miles and switched to synthetic oil, I don't think you'll experience an oil-related engine problem.