This may be a dumb question, but my 1984 Ninety Eight has had some dieseling issues, and it came to my attention that I'm not exactly sure what rating fuel to run
I've heard people say 87 and 93, but which one is the actual rating it ran on

 

Your EGR might not be working if you get detonation under light throttle. The passages within the intake manifold can clog up too.

 

Dieseling on a 1980s 307 is frequently caused by a malfunction in the Idle Load Compensator (ILC) system. The VIN Y motors used a vacuum-actuated plunger to limit throttle closure, but it works backwards from how you think it should. When there is no vacuum, the plunger extends, increasing idle speed for conditions like A/C on or PS pump under load. When vacuum is applied, the plunger retracts, reducing idle speed and reducing the likelihood of dieseling. Now think about that for a second. When you turn the engine off, you loose vacuum, so the plunger extends, which increases throttle opening and thus increases the chances of dieseling. Olds got around this by using a vacuum reservoir ball that was closed off from the ILC by a normally open solenoid valve. With the ignition on, this solenoid valve closed, maintaining vacuum in the reservoir. The intent was that when you turned off the ignition, that solenoid valve would open, allowing the vacuum in the reservoir to hold the ILC plunger in the retracted (lower idle speed) condition long enough for the engine to shut off without dieseling. That works great when the system is brand new. Any leaks, through cracks in the vacuum hoses, wear in the ILC, or worn seals in the vacuum solenoid, will cause the vacuum to be bled off too quickly, and dieseling ensues.