On page 231 the 2017 Volt Owner's Manual states If TOP TIER Detergent Gasoline is not available, one bottle of GM Fuel System Treatment Cleaner added to the fuel tank at every oil change, can help. GM Fuel System Treatment Cleaner is the only gasoline additive recommended by General Motors. It is available at your dealer.
I'm wondering if this is not another "cut and paste" from an ICE perspective. Two years between oil changes is potentially a lot of miles in CS mode for a Gen1. I've got 9300 miles on my last oil change done 3 months ago and my "Oil Life" display is at 62%. I'm driving a 2014 Volt without Direct Injection. The Gen2s with DI would NOT benefit from a more frequent use of the Fuel Treatment Cleaner as the valve heads and stems aren't being continually bathed in a flow of gasoline vapors to do that job!
When I went to the dealer for replacement of the transmission fluid at 54,000 miles, prior to my latest cross country road trip, the mechanic did a multi-point inspection (free of charge). He recognized that I had replaced the air filter and had replaced the engine oil. He - through the service writer - recommended that I do a cleaning of the intake manifold (I paraphrase 'cause I don't remember the term that was used) because DI engines need that maintenance. I pointed out to him that my engine wasn't DI, and had injectors squirting into the manifold outside the combustion chambers.
My recommendation to all Gen2 owners, when your service writer recommends cleaning the manifold, you should seriously think about it. He is not blowing smoke.