Well, thicker oil is going to be harder to inject which means the actual injection / ignition may be a little bit later, but thicker fuel is also going to cause the internal pump pressure to be higher, which will advance the timing. Pump timing works off pump pressure, which is a function of the flow rate into the pump (from the vane pump inside the injection pump, and a function of pump rpm) vs the flow out of the pump through an orifice - thicker fuel flows out slower, hence higher pressure and more advanced timing.
IIRC, this is one of the major design features of these type of injection pumps - some degree of tolerance for different fuel viscosity.
Now, that being said, it's still only going to work within a certain range. Going too far outside the design limits and you are going to get timing variation. I would expect that the timing advance from thicker oil would override the retard, so the net effect is going to be more advanced timing, which would make a noticeable difference in sound and drivability.
Personally, I would be pretty worried about injection system health - the higher the viscosity, the harder that pump is working to inject the fuel, and you're talking about a large amount of force. Try pumping gear oil at the same rate as 10W30.... takes a lot more force. If it's enough of a difference to be making noticeable timing difference than I'd say it's enough to cause worrisome internal pump wear/damage.
