Originally Posted By: CATERHAM
That was a very informative thread but I do have a question.
As I understand it, AW/FM additives such as ZDDP and moly are sacrificial, in so much that during boundary/mixed lubrication what's plated onto metal substrates is depleted as it's worn off since they are no longer in a reusable form?
How does the usefulness of the initial AW/FM additive concentrations of fresh oil play out as mileage is racked up on an engine? In other words, is there a set depletion rate for AW/FM additives?
For example, take a typical energy conserving motor oil with 600 ppm of phosphorus and 100 ppm of some organic moly type. How effective are the AW/FM additives at 3,000 miles, 5,000 miles, 10,000 miles and 15,000 miles vs the virgin oil?
And how would this oil when fresh compare in it's ability to protect vs an oil with higher additive levels that had already accumulated say 10,000 miles?
As long as the additives are present, and they are still active, then it makes no difference. But there is no uniform depletion rate as operating conditions play a big factor. This is why phosphorus retention has been such a focus in the GF-5 & GF-6 developments. However as explained below, depending on how the formula is put together the results could vary, especially if the moly becomes dehydrated. I'll have to look further into it, but I'm not sure if much comparison testing has been done on the effectiveness of the FM/AW on oils which are aged to various mileage intervals.
Quote:
I think someone once posted that organic moly has lost most of it's effectiveness by 5,000 miles?
That was me (several years ago):
Moly content in oil typically comes from MoDTC (molybdenum dithiocarbamate) or MoDDP (molybdenum dithiophosphate) and it reacts with surface metals to form Molybdenum disulphide (MoS2) which is what creates the sacrificial wear layer. However over time the exposure of the molybdenum additives to oxygen changes the surface layers to Mo03 which is what is referred to as oxidative dehydration, this reaction takes time and so that is why the good fuel economy achievable with MoS2 in oils is hard to sustain over long service periods. (Basically oxygen + moly = very little wear protection). Water can also contribute to the deterioration of MoDTC and it can be difficult to see the effects of Moly continue beyond the 4000 mile mark. Using antioxidents can help prolong the effect that Moly has and so does our friendly ZDDP. So depending on what the additive package has blended with it Moly content (MoDTC) can still be very high but be basically inactive due to oxidative dehydration.