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I don't think it hurts to use synthetic, other than some say it lengthens the time to fully break in the motor. FWIW, I use Castrol GTX in my 4s. Coincidentally, when I bought my oil filter, the mechanic also recommended that same oil. He must have felt strongly about it because I didn't ask for his advice.
I normally use Lubrimatic LU oil, and sometimes use regular name brand gear oil. Your owners manual will indicate the minimum API requirement for your crankcase and LU oil.
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