One disadvantage to changing an old bike, car, etc. over to synthetic is that if you have a compromised seal somewhere that's being held shut by the smallest amount of sludge, synthetic oil tends to clean away the 'sealant' and accelerate the leak. I experienced this on my previously non-leaking 1978 Chevy Monza, which had a sudden rear main seal leak after switching to syn.
If through the magic of a crystal ball you happen to know that all your seals are in excellent shape, synthetic would be fine. Just don't be surprised if the engine starts consuming oil. That's the other disadvantage that seems to be hit and miss from one engine to the next. Syn can find its way past worn rings where conventional didn't. My 1986 Civic Si was a prime example of this. I switched it back to Castrol GTX so I wouldn't have to top it off between oil changes.
Sometimes the consumption slows and stops after the switch, sometimes it continues on to the end of time. My 2004 Troybilt mower with a 6.5 Briggs consumed no oil on straight 30 conventional, but consumed an alarming amount of 5W-30 and 10W-30 synthetic. 10W-40 syn slowed it, and 20W-50 syn stopped it. It depends on how well your rings have stood the test of time.
Personally, I prefer syn for a hard working motorcycle engine...especially air cooled. It resists coking, shearing and thermal breakdown better than any conventional. Unfortunately, if it causes a compromised seal to start leaking, there's no going back. You live with the leak, or replace the seal. Consumption can most often be reversed by going back to conventional or thickening up the syn. If you're already using 20W-50, of course thickening wouldn't be advisable.
Good luck with your decision.
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