I have a recently-running 1999 Metro 6/1. It's hooked up to an ST-5 generator head. I'm using a thermo-siphon and 20 feet of baseboard heating cores as a cooling system. I have machined an adapter for an automotive thermostat (195 degrees) be be placed in the head to regulate temperature. The cylinder head heats to about 190 before the glycol starts flowing and holds a temperature between 193 and 205 for the entire time of the engine's running.
Here's the question: no matter what load I put on the engine, everything from idle to a continuous 3,500 watts being pushed by the generator, hours at a time, the water remains nearly constant in temperature, and heats quickly at initial startup because of the thermostat. I'm running just under a gallon of Rotella SAE30 oil in the sump. No matter the load, no matter the time the engine runs (often 8 hours or more at a time), the base of the crankcase never exceeds 130 degrees, which tells me that the oil never exceeds 130 degrees. It takes quite a wile (well over an hour of hard running) to attain this temperature.
I've always done well by the rule of never starting an internal combustion engine unless I plan on getting the oil good and hot to drive out the moisture byproducts of combustion, and in my mind, that means getting the oil above 212 degrees for a reasonable length of time. In this engine, however, that does not seem possible, and concerns me about the oil life and the buildup of acids and other unwanted chemistry.
A secondary question with this is that if the sump always stays relatively cool, should I consider putting a 15 weight oil, or a multi-viscosity oil (15W-40?) for winter? It's not uncommon for me to want to run this engine at temperatures well below freezing, and since the sump never gets hot, I'm seriously wondering about the effective splashing action of honey-thick oil in there.
What has experience taught all of you who have had their engines run far more hours than I have yet to put on mine?
I made a (crummy) video of the engine to show the guy that I bought it from that I got it running, and it shows the cooling system, mounting, generator, etc, in case you're curious about the layout or perhaps I've made some fundamentally poor design choices that I need to address. Feedback and your own experience welcomed.
https://www.youtube.com/watch?v=LPBrGu09J9UThanks!
--Justin