This is not true. The post Newtonion limit assumes weak gravity and slow speeds. The 1PN formalism describes this limit. However, the PPN methodology has been extended well beyond 3.5PN order, allowing consideration of fairly strong gravity and relativistic orbital motions. See section 4.1 of the following for minimum PN levels required for different cases:PPN goes too far. If i understand correctly, it even exclude special relativity effects.
I'm interested in gravitational effects.
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