Originally Posted By: JeffKeryk
From:
http://www.machinerylubrication.com/Read/28576/comparing-gasoline-diesel-engine-oils-
Diesel engine oil has more additives per volume. The most prevalent are overbase detergent additives. This additive has several jobs, but the main ones are to neutralize acids and clean. Diesel engines create a great deal more soot and combustion byproducts.
It is received wisdom that overbased metallic detergents carry out a 'cleaning' function in engine oil. In my experience, this just isn't true.
To understand what an overbased detergent actually is, try and imagine a beaker which contains one third powdered chalk (calcium carbonate) and two thirds of mineral base oil. The resultant gritty, opaque, gooey mess might not be something you'll want to put into your engine if your objective is keeping things clean! It would also block your oil filter in minutes.
Now imagine you can remove from the beaker a bit of the base oil, and replace it with some 'neutral' calcium alkyl sulphonate (often referred to as 'soap') which, through the magic of colloidal dispersion, breaks up those particles of calcium carbonate to the point where they're soooooooooooo small, the mixture becomes transparent. That's essentially what an overbased detergent is.
The primary purpose of the calcium carbonate in detergent in to neutralise acids (just imagine putting citric acid in your kettle to react with limescale...same thing!). The primary purpose of the calcium alkyl sulphonate is to colloidally disperse that inorganic base (hence the term 'overbased'). Now some folks will tell you that the same calcium alkyl sulphonate will keep those acid-base reaction products dispersed in the oil and help prevent deposits dropping out. I think at one point this was probably true but in today's environment, I'm not sure it is and if it still is, then it's a very minor effect.
Hope that helps...