I have a 1972, 240z with approx. 95,000 miles. The car has a high mechanical decibel level when the transmission/clutch is engaged. Other than that, the car seems to be running fine. I have been thinking about using synthetic engine oil and transmission fluid. I know this has probably been asked before, but is there a downside with synthetics on the 240's engine/transmission seals and gaskets.

Depends on what you mean by a high decibel level when you engage the clutch. Is the noise a squeel? If so, it sounds more like the throw-out bearing may be going bad.....

Synthetics in an old engine are probably a waste of money, as they won't cure any wear problems in the engine.

Synthetics like Red-Line MTL in the transmission might help a bit, as they may help with sticking synchronizers and improve the shifting to a point, but you should make sure the trans is not in need of any other repairs by checking the old fluid for metal shavings before you re-fill it with more expensive fluids...

A synthetic diff lube may help control a heat problem, but only if the rear is in fairly good shape, same as the transmission, check the old fluid carefully.

The synthetics do have a slightly lower viscosity than regular petroleum based oils, while giving the same protection.

The noise is more like a "whining" sound. At times, it is intermittent. I had the pinion bearings in the differential changed out and this had a limited positive affect. On a side note, should the dashpot needle for the SU's be flush with the bottom grooved surface or flush with the overall bottom surface?

I have gear noise on deceleration or lack of throttle input, which is supposedly incorrect rear end gear backlash, but I haven't finished troubleshooting.