964 C4 cars equipped with PDAS (Porsche's traction control system) have rear differentials that require the proper LSD additive in the gear oil. The 'rear differential feels locked up' complaint in service records is commonly the result of wrong or stale fluid, not a mechanical problem — Zuffenhausen-style invoices flag this with the explicit note that the diff 'can be left as-is unless you want rear differential locking capability'.
C4 PDAS rear differential — LSD additive required
Porsche 911 (964) · mechanical