I have a 1985 Universal M-18 and send oil out for analysis on every change. The most recent analysis indicated glycol in the oil.
I haven't noticed a decrease in coolant, increasing oil, or bubbles in the overflow
so the issue is probably small.
I pulled and rebuilt the exhaust riser this winter after discovering a leaking exhaust flange gasket. Maybe far fetched but I could imagine how a pressurized freshwater loop might leak past the exhaust flange and back into the engine. Hoping this fixes the issue.
Another culprit might be the exhaust manifold gaskets. I noticed potential puffs near the exhaust gaskets when there was spilled coolant burning off after bleeding the freshwater loop.
Of course this could be a head gasket issue.
Current plan of attack is:
1. Buy/rent a block checker and see if exhaust gasses are present in the manifold
2. Retest oil after riser rebuild
3. Keep oil clean with short oil changes (10-20 hours)
4. Replace exhaust gaskets
5. Replace head gasket (would do intake and exhaust gaskets if not done before)
Any other thoughts might be helpful. After reading the service manual, a head and other gaskets seem doable.