I know this is a Merc board, not a Volvo board, but am looking for advice everywhere.

I was having trouble starting my 85 740 turbo wagon w/ 162K on it so I decided it was time to change the plugs. Wow were they bad. starting at the front of the engine, the entire plug was covered with carbon, which decreased as I worked my way down the block. The fourth and final plug had little carbon, but had oil on the threading. Is this a sign my engine is on the way out? It had been about 20K miles since the plugs had been changed.