Being a 603.971 it can only be bent rods

Pretty obvious from the smell whether it's unburned fuel or oil... or coolant

The best of us (aka gsxr) have been victims of bad or improperly installed valve seals.

If there's an accumulation of grime or wet oil in the aread around the #1 exhaust runner, it could be a bad head gasket. The gasket doesn't do a good job of sealing the oil feed for the head. Not for very long, anyway. Oil seeps past the gasket externally and to the #1 cylinder. Why it only manifests after sitting for a few days is beyond me.

Sixto
87 300D