Not grease, it is oil, if I remember right. What little is required comes through from the timing cover via the driving sprocket spindle and from hence to the chain. Excess drains down and is fed away through a drain via a small pipe.
"Grease nipple on the left side top"?? If you have a grease nipple on the "left side top" it is a later modification, oil for the timing chain comes from the engine and then drains out the bottom of the case.
I have never seen a grease nipple on the timing chain case.
Anything there is indeed a modification.
The engine is designed to leak some oil past the timing pinion and to drain any excess through the pipe at the bottom.
I guess both grease or oil will do but grease is not necessary on a "normal" engine.
If you put in grease I guess it will have to be a lot!
As mentioned before it may melt while hot and clog up the drain as it cools.
Whether that is a problem, I do not think so.
The grease will however be diluted by the oil which will come from the pinion anyway unless that passage has been blocked by a deliberate seal.
I do wonder if that is the case.