Our bikes are almost 'pre-historic' in terms of the engine design. Especially the top end, where the ZDDP will benefit most.
Off hand, I can't remember how much ZDDP to use, but it's the equivalent to a couple of caps full per oil-change.
The gurus on here will know better
The main thing is to use an oil without friction modifiers (for use with a wet clutch) or your clutch will start slipping.
I know your area well
My son lives in Scarriff, Clare, I spend some time every now and then in Cloughjordan, Tipp, I used to work at Crannagh castle, and used to house-sit winters in Balinure/Horse and Jockey area.
I know the pub in Moneygall which is where the Dublin/Limerick bus stops...... and where a certain man called Obama has links
Next year's Bikefest sounds good to me!