Once upon a time, many, many moons ago, they made a grease gun with a special tip for those cast fittings. Actually, two or three types.  Best one had a rubber insert in the tip that simply pressed against the fitting.  There was a similar one that instead of the rubber , simply had rounded/concave end that likewise pressed against the fitting.  Those types were hand-packed with grease to fill.  Those were an easy after-the sale-accessory item along with a can of grease when closing the deal on a new motor.  Then there was the one that had a similar tip and screwed to the end of a tube of grease.  That one was nice, but didn’t work well.

I’ll look when I go out to the shop to see if I still have a gun with one those tips and report back if I do.  The one with the concave end would be an easy one to reproduce.