The recommended octane rating for a 1975 Cadillac engine depends on the specific engine it has. Cadillacs in 1975 used large displacement V8 engines. Most likely, they called for
regular unleaded (87 octane), but some high-performance options *might* have recommended mid-grade (89 octane).
You absolutely must check your owner's manual. That manual will specify the correct octane rating for your particular 1975 Cadillac model and engine. Using the wrong octane can damage the engine.
