Yes, synthetic oil probably helped a lot. I'd do a compression test and maybe stick a boroscope into the cylinders if I had one before tearing into it. If it isn't hydrolocked and it starts and runs, you could also look for bubbles in the coolant overflow tank. When the headgasket failed on my Landcruiser, I could smell the hydrocarbons in the overflow tank and see the bubbles even though it was running fine.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13