I am Giles' apprentice here at Performance Diesel Injection, just so that you know. We often have different opinions on biodiesel / renewable fuel matters, but I'm not contradicting him either.
There are a whole bunch of reasons why Bosch doesn't recommend anything above B10 - Giles and I were just having this discussion, because when we see a "Biodiesel" pump, it's because it's gummed up, pitted, rusted and destroyed - usually by bad biodiesel or biodiesel that wasn't properly created or prepared, but not by the simple fact of running on biodiesel. He isn't really a fan of biodiesel, and really disliked it until just lately, now he's a little more neutral. I on the other hand, am a big fan.
The simplest answer as to why Bosch doesn't "recommend" greater than 10% is that nobody has done long-term, life-cycle laboratory tests on the effects of biodiesel on their pumps. Since they can't guarantee that it's going to function properly, they don't try to.
The longer answer is this: in 2005 (most recent stats I could
easily find), we used 56,000,000,000 gallons of diesel fuel. The National Biodiesel Board is hopeful that we might be able to produce a grand total of as much as 300,000,000 gallons of biodiesel
for this year. The upshot of that is that now, after half a decade of development of the biodiesel industry, it could account for as much as 0.5% of the diesel fuel consumption of the United States, IF it is all burned domestically and none is exported. That doesn't exactly sound like a sufficient market share to bother with the level of testing that would be necessary, right? And that is the biggest reason right there. There are also a variety of feedstocks (oils like canola, soy, waste vegetable oil etc.) available, of varying qualities. There are many smaller manufacturers, as opposed to a handful of more-or-less consolidated major players in the petrodiesel production business. That presents greater variability in fuel quality, as well as a larger possible number of potential points of error or negligence.
Biodiesel remains the product of an industry which is in its infancy, and it operates on a completely different scale than the petroleum industry, so it's not really worth the time and energy investment to test it, particular given the confounding factors of different feedstocks, changing test specs for the fuel, etc.
Aside from that, though, Bosch doesn't warranty ANY work caused by fuel problems, no matter what that problem is. The care and feeding of your engine is your responsibility, and if they can demonstrate that the fuel you gave it didn't conform to the ASTM spec for diesel fuel, then they can legitimately refuse warranty coverage. If you were to try and set up your own fractional distillation tower, get some crude oil and refine it into its components, then use that diesel fuel, you would still not be covered.
But biodiesel itself isn't a problem, the problem is the fact that there are such variations in quality and the fact that Bosch (or other manufacturers) can't say "Yeah, use it, it'll work fine!" because then somebody will make a bad batch and blame the manufacturer when their engine fails.
Does that all make sense? There is a difference between what the manufacturer acknowledges will work and what the manufacturer is comfortable guaranteeing. But the bottom line is that properly made biodiesel which conforms to the ASTM spec for biodiesel (
ASTM D6751-07b) will not harm your pump. Properly made biodiesel, meeting these specs has better lubricity and higher cetane than petrodiesel. It offers equivalent or reduced emissions in all areas except for NOx, which is still unclear - in stationary applications and tests, NOx from biodiesel is increased. In driving conditions, it may be equivalent or lower, but those tests have yet to be done. At the same time, Biodiesel made by someone who is negligent or fails to understand the processes involved, or otherwise doesn't / cannot adhere to production practices which yield such a substance, will damage your pump in short order, just the same as bad diesel fuel that doesn't meet the required spec (
ASTM D975-07b) will damage your pump and/or engine in very short order.
So is biodiesel destructive? No. Is all biodiesel the same? No. Should everybody make use of it? Not necessarily, because quality is still loosely enforced and hard to verify. But will it destroy your pump? No, and that's not what the manufacturers are saying - all they're saying is that (I'm paraphrasing, not quoting) "Biodiesel blended in no more than 10% will not adversely affect any characteristic of normal diesel #1 / #2, nor will it adversely affect your pump's operation in that proportion, and they are so certain of that that they won't reject your warranty out of hand for using a B10 blend."
Hope this answers your question without making it sound like Giles and I are saying two different things. We both stick to the manufacturer recommendation in terms of what we can support, but this hopefully sheds more light on why the manufacturers take this kind of a position relative to biodiesel.