Synthetic reprocutions?

[TRMatthews@domain.elided]

Hello all,
I am going to switch from Castrol GTX to a synthetic. My question to the 
digest is this: Because it is nearly impossible to drain all the "regular" 
oil out of the motor, is it harmful to add the synthetic? Also, will the 
remnants of the previous oil prevent the synthetic from properly bonding to 
the metal in the motor? Any ideas or thoughts to this matter? The car has 64 
thousand miles on it if that makes any difference to the equation....

Thanks for your thoughts,
Todd

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