For a command to increase something, should instructions refer to the "+" key or the "=" key?

Our software uses the =/+ and -/_ keys for opposite tasks. For example, increasing or decreasing a value, or zooming in or out. No modifier keys are used.

Here is the conundrum:

• If we refer to it as the "plus (+) key," users might think they have to hold Shift.
• If we refer to it as the "equals (=) key," we lose the intuitive dichotomy against the "minus (-) key."

Is there:

• Research that gives a reason to prefer one form over the other?
• A mention of this in an authoritative style guide that establishes a convention?
• A good example of this in the wild?

Edit: Concerns about other keyboard layouts are noted, but for the purposes of this question, I'm only asking about US keyboards.