I have an 03 silverado with a manual transmission, (mg-5). The dealership say it takes a synchromesh fluid. i am tryiing to find out if it is acceptable to use a75/90 synthetic. any input is greatly appreciated.

Synchromesh usually means oil minus damaging EP additive, in this case you can try out Chevron Delo gear oil which is EP free and uses Boron instead and is sythetic. You can also use Mobil-I synthetic gear oil.

If it concerns the NV3500 gearbox, it's recommended to use a GM partnumber GM 12345349 (which is claimed to be a 'special gear oil'). If it concerns the NV4500 gearbox, GM's original recommendation is a GL-4 75W-85. Here you could safely use a 75W-90, if you also keep the comment of Gurkha above into account while selecting a product.

Historically most synchromesh gearboxes used
SAE 80 GL4.The use of SAE 80W-90 GL5(higher viscosity and more additive)caused gear change problems. Rationalising to 75W-90 GL5 only partially helped the problem in 5 speed boxes(to cure hot rattle).
Manufacturers of 5 and 6 speed synchromesh passenger car boxes then required lubes to overcome cold change and high temperature rattle along with gear change feel, hence in Europe, Japan and Korea cars this led to the profusion of gear lubricants GL2 to GL3/4 and various viscosities 75W-80W to 75W-90. The use of Synthetics typically PAO, not likely to affect frictional properties but of benefit for viscosity shear stability and extended lubricant life.In answer to your question don't forsee any great problems other than oil likely to be more slippery(GL5) causing minor synchro problems.