Some of this is due to the additional viscosity resulting in higher pressures. If effect changing the way the pump operates.
More than you may think.
For those not familiar with the Bosch VE-style IPs (injection pumps), the automatic timing advance is purely hydraulic: there are NO mechanical flyweights to advance the injection timing at the RPMs increase.
The lift pump is built-in to the front of the IP. It's a vane pump, similar to most power steering pumps. It serves two purposes: it pulls fuel from the fuel filter, and it pressurizes the IP's internals. There is a timing piston that reacts to the IP's internal housing pressure and it acts against the cam plate over which the plunger rollers move, changing the timing.
This is all background: the Bosch VE-style IPs vane pumps have a pretty high wear rate, and gradually are unable to properly pressurize the housing, though they'll still work well enough to pull fuel from the tank etc. This means that the effective IP timing becomes retarded
at the higher RPMs only. From my testing, this can't be detected with a Tach-n-Time, Ferret, Snap-On MT257, OTC, etc. piezo pickup injection timing setup, and probably not with an optical glowplug-hole pickup either, because Bosch engineered the VE-style IPs to not advance unless under load, and even then they have a feature they call Torque Control or something that retards the timing even under those conditions.
Summary: higher viscosity fuel may change the timing of a worn VE-style IP in a positive direction and may be a variable in how well an alternative fuel works in an old diesel having a worn IP.
Most of this info comes from the
Bosch "Distributor Type Diesel Fuel Injection Pumps" softcover technical "Yellow Book" from Amazon. It is slim, at 134 pages, but fairly dense. It is chock-full of illustrations that are better than average. It is also, unfortunately, a starkly translated manual and as such must be read with that in mind; it is
very easy to misunderstand the intent of a statement, and this will lead you to incorrect conclusions that can be hard to bend back to reality later.
Here, I reproduce three pages (click on any image for larger):
(I have some of the text OCR'd in
this post on my site.)