I have the following layer stack up
Layer 1-2-3 are tightly coupled
Layer 4-5 are tightly coupled
Layer 6-7-8 are tightly coupled
Total thickness is 120 mils
If I transition from either 1 or 3 to 6 or 8, is a via or stitching cap required in order to keep returns happy ?
I'm thinking I would because of layer 4.
The requirement for a stitching capacitor is covered in this diagram: -
This isn't quite the full story because decoupling caps from Vcc to GND are also needed (and will normally be present for the chips) at both ends of the signal.
As far as I can tell, in the question, any signal layer is adjacent to a GND layer so this means stitching caps are not required. Stitching caps couple the signal return current between planes that are not at the same potential i.e. Vcc and GND. Both (usually) behave the same way for AC return currents.
Regards stitching vias, it is always good practice to couple multiple ground planes together at regular intervals so that ground return current paths do not form a significant "antenna area". In the picture above, if the Vcc layer were in fact another ground layer then it is advisable to have a stitching via close to where signal1 changes layers.
Having said all of that, plane to plane capacitance can be a significant reason for not using either method but, if the frequencies involved are high (50 MHz upwards I would say) then care must be exercised. Always check the natural resonant frequency of stitching capacitors - smaller values are better!
High frequency return currents want to follow the close to the signal currents and should be allowed to do so.
So yes when a signal changes reference plane you should stitch the two reference planes together close to where the signal changes reference planes. If the two planes are both ground (or both the same power rail) you can connect them directly. If one plane is power and the other is ground then you need to have a capacitor so they are connected together at high frequency but remain separate at DC.
In your case the two layers you are using as signal references are both ground, which would call for stitching vias but not stitching caps.
