Haven't met a Spridget yet that didn't leak oil whether conventional or synthetic. Synthetic just seems to find the places it wants to escape from better than conventional. Has to do with molecule size.
I've been fighting rear main leakage on Spridgets for 35+ years. Nothing really works 100%. I'm now doing some machining in the rear main cap that assists the draining aspects back to the sump. Verdict still out on that one. Seems to me that most of the leakage happens on engine shutdown when the accumulated oil in the galley ways overwhelms the two small drain holes out the rear main bearing cap. Probably has to do with the off set nature of the drain holes and their size.
Use something 20w50 that has the ZDDP level close to 1200ppm. Off the shelf SM rated street oils from the big oil companies won't cut it. They are down around 800 ppm. You might have to use something specialty or dedicated for racing purposes. Zinc and phosphates level are being lowered to extend catalytic converter life. RP is fine, I'm partial to Valvoline products and use the racing oils. Need to be more frequent on changes with some racing oils since they don't have all the detergents and corrosion inhibitors. Check the product web sites to determine oil ratings, additives and ZDDP contents. You might also consider some of the 15w40 diesel oils. Most have ZDDP levels around 1100 and have some additiional additives that are helpful. Do some research. One of the ones like Delo, All-Fleet, Rotella or Del-Vac might be attractive.
As far as how much oil, The link will give a capacity figure, but what I like to do is to put in 3 quarts, start the engine, let oil pressure settle, run just a bit and then shut down. Check the oil level and top off. That way you aren't trying to measure a "dry" engine. That is oil hasn't puddled and accumulated everywhere it is going to. Also factors in oil cooler and lines if installed. Checking after shut down gives a better indication of a full sump. At least to me. Be sure to fill the oil filter first. Almost to overflow and then install. That way the oil circulation and pressure build up time is shortened.
HTH,
Mike Miller