When VW ran into emission certification problems with their Touareg diesel earlier in the year, it cast doubts on the viability of an emissions-reduction technique called "urea injection". One of the main drawbacks of diesels is an increase in the emissions of nitrogen oxides (NOx), one of the nastier pollutants and a result of the high temperatures and localized areas of lean combustion inherent to diesel operation. Urea injected into the exhaust stream of a diesel decomposes into ammonia, which then lends towards the breakdown of NOx into oxygen, nitrogen, and carbon dioxide. The issue isn't the effectiveness of the technique; it's the maintenance aspect of the whole arrangement. Current Mercedes diesels that use the technique require a urea refill every 10,000 miles or so; the EPA worries that consumers won't follow this schedule and emissions non-compliance will result (they require that emissions systems will remain functional for 10 years or 150,000 miles). So what's the problem? Install a low-urea warning light (I don't think I want to know what the symbol for that is going to look like), offer the stuff up for sale in parts stores, and get on with the task of saving some oil. Ah, but companies like Mercedes want to make the system "invisible" to owners. With oil changes going out to God-knows-how-long and many other fluids considered to be "lifetime fills", it's about time we put the owner back in charge of a simple maintenance task or two.