Historically, high ash has been a concern relative to combustion chamber deposits in heavy duty diesel engines. For years, the magic number was to have a 1.0% ash content. That has evolved over the relatively recent past where ash content was not as strong a driver as TBN due to extension in drain intervals. Thus, most heavy duty diesel engine oils are now typically 1.3% ash and the associated TBN is higher as well. I honestly would not worry about the ash level of the commercially available 15W-40 oils in your vehicle relative to whether or not you'll see increased deposits. For perspective though, the
ACEA standards for light duty diesel engines, for instance, allow for between 1.3 and 1.8% ash (see table 2b).
PS - If you're really into it, here's a
paper you can order from the SAE. There are other SAE papers that are more specific to the comments above but I don't see them as available online.
[ April 03, 2003, 05:10 PM: Message edited by: 68redlines73 ]