what is considered a good oil pressure at 65mph and 40 mph in a i991 mx-5 150 000 on the clock

As engines wear the tolerances between moving parts increases. How much this increases depends on several factors including frequency of oil changes.This can lead to lower oil pressures. With 150k you would have some normal wear. You will have a slight variation in pressures depending on engine rpm. If the lowest you go at idle is 30 you are fine. With an engine of 150k if you have over 30 all the time your engine is getting enough lubrication. You can run a high mileage oil to boost the pressure. Also if you notice quick variations in pressure you might want to manually check oil pressure. Its a possibility that your oil pressure transducer is becoming erratic.

If I read your question correctly, you are normally seeing 30psi @ idle, but as little as 10 between 50 and 60mph. Is that correct? If so, I might pull the sender and clean, or replace it. And as suggested above, put a manual gauge in the system to verify readings. If you are still see erratic readings you may have several problems. 1) Oil sludge in the pan which is clogging the oil pickup. 2) Build up (sludge) in the oil return galleys so oil is returning to the pan too slowly. 3) A weak bypass valve in the oil pump. 4) Bad oil pump or pump drive. 30psi @ idle is good, exceptional in fact. 10psi @ speed is not.