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What design or component of an OMC electric shift requires that only a special lubricant be used?

That is a good question for a forum that concentrates on older OMC outboard engines. I don't think the OMC electric shift gear case has been made in many decades.

My best guess for OMC to specify a particular lubricant for an outboard engine gear case that had electrical actuators: the fundamental difference between an electric actuator in a gear case and a mechanical actuator is that electrical current will be flowing in the gear case. On that basis, I would expect that the electrical conductivity of the lubricant would be a consideration. In order to not interfere with the electrical circuits in the gear case, the gear case lubricant would probably need to be a very good non-conductor. I would further guess that some formulations of lubricants for use in an outboard engine gear case might have additives in the thick oil that are electrical conductors.

A google search I made on this topic revealed an interesting presentation by a mechanic who was very familiar with the electric gear case. He comments that need for a special oil was related to the mechanical design of the clutch mechanism, which required a very specific oil to work properly. You may find this presentation to provide more answers to your question:

OMC electric shift lower unit
https://www.youtube.com/watch?v=H0hYW6XJfpc

Watch particularly at this point in the presentation
https://youtu.be/H0hYW6XJfpc?t=170

when there is a mention of use of APG80 Hydraulic oil.

Many outboard engine manufacturers place restrictions on the lubricant to be used in their gear case. That many decades ago OMC specified a particular lubricant for a particular outboard engine gear case is not unusual.

Wow, electric shift, those are from the 1970's.

In an electric shift lower unit, the shift solenoids are in the lower unit bath of lower unit fluid, much like the shift solenoids of an automatic transmission. There are 2 reasons for using Type C or equivalent fluids in those lower units:

1. The additives in some lower unit oils act like a conductor and not a true dielectric of type C fluid and cause the lower unit not to shift. The shift solenoid connectors are open metal connectors exposed to the oil and the conductivity of the (non type C) fluid can cause "shorting" between the connectors rather than the solenoid internals getting the voltage needed to operate.

2. Type C is a slightly ligher viscosity.

D-

Very interesting and logical if they needed to be something non conductive.

If these OMC electric shifts date back to the 1970's (from what I read 1972 or 197373 or to 1978 for sterndrives) it is impressive that Mercury still notes this on gear oil being sold in 2022.

I had a 1972 Evinrude 125-HP outboard engine. I believe that was the last year for the electric shift. That was also the first year for electronic ignition. There were solenoids inside the gearcase that selected the gears. The default position was forward. That way if the electrical system failed the motor could still be started in gear and moved normally. When I filled the gear case, the oil appeared the same as any other oil. But as mentioned above, it was special to these units. I did not know about the concern for electrical conductivity .