The reason for using conventional over synthetic at the beginning of an engine's life is that synthetic is too 'slippery' and wouldn't allow proper friction between the various moving parts inside your engine.
50 hours is perceived as a reasonable/good amount of time for breaking in your new engine.
The fact it is spread over 2, or even 3 years is irrelevant. I don't know what your habits are, but personally, I'll be changing engine oil and filter at the end of each mowing season (fall). And most likely, I will not even put 50 hours this Summer as I just started to use my 48". I was using a 42" prior to that.