I had a 4-108 on my Freeport 36. The engine only had about 800 hours on it when purchased but when I changed the oil I'd get carbon on my hands that would take a couple of days to remove. The oil was black within 8 hours. The 6-354 we have on TD runs really clean. After 100 hours it's still somewhat transparent and not at all like the 4-108. The 6-354 has at least 6,000 hours on it and is 30 years old.

Only thing I could come up with is the 4-108 was abused when new. Trouble is, it's difficult to abuse something with so few hours on it that it turns the oil black so quickly.