Well the OP engine builder most likely meant to not use synthetic with his FT lifters, not void the warranty because it causes leaks on a new engine.
I have switched from conventional to synthetic oil countless times with no leaks... unless a gasket was bad or other problem with a seal. Have also switched back & forth between the 2, both types of oil are compatible with each other and can alternate between the 2. Synthetic blend oils are a combo of regular & synthetic.
Cant speak for other builders, just that Ive never heard about voiding a warranty on a rebuilt engine related to leaks from synthetic oil. Some say or said in the past synthetic oil was bad for FT cams & lifters because what I mentioned above, but today there are numerous brands of synthetic oils geared towards hot rods & classics with FT cams. I have used & use full synthetic or blends in a FT cam pontiac v8 and other engines with no problems so hard to say whats right or wrong in that regard.
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