Funny you mention the MX5. I have a 124 Abarth Spider. The gearbox in this is essentially out of an MX5 NC! When cold, the gearchange was terrible - far worse than the transaxle on my 75!! Apparently, this is a common complaint re this box. Supposedly fixed in the MX5 ND. I tried a number of different lubricants in the 124 box. Nulon 75W/85 made a reasonable improvement, however as the 124 is almost my daily drive, very cold mornings were still an issue till the box warmed and the oil thinned a bit. Final fix was Penrite Pro Gear 70W/75. As I recall this oil has a very high viscosity index so it won't thin much when hot. Important for a relatively thin oil - don't want it thinning much further!
Granted the "slipperiness" of an oil has an impact on synchro performance, however methinks viscosity also plays a major role. I prefer a full synthetic as these generally have a higher viscosity index (less affected by heat) and usually (but not always) a higher film strength. I'm not sure synthetic oil = a more "slippery" oil or a mineral oil is less "slippery" than a full synthetic.