It's not so much don't shoot heavy pellets as it is don't shoot really, really heavy pellets designed for PCP's. In most instances, they won't work very well anyway. Kind of like taking lead bullets and cutting a cross into the tip. Sounds awesome but it seldom produces anywhere near positive results. So, if your gun likes a pellet on the upper end of the weight scale, that's fine. Maybe a bit quicker wear on components, but that's all. Average lifetime of springs and seals is exactly that - average based upon standard weight ammo. Guess if you went really light, well, we know what PBA's can do to a gun.