I have seen occasional posts on the T&T list and on the GB forum from people who said they switched their FL120s to synthetic oil. They have cited no problems and listed benefits with less smoke being the primary one and probably other things, too that I don't remember. In most cases, while their engines sometimes had a fair number of hours on them when they switched, they had run the engines on synthetic oil for a few hundred hours at most at the time of their posts. I've not yet seen a post from anyone who's run an FL120 on synthetic oil*for many thousands of hours or many years
But all the people we know or have talked to who are familiar with the FL120 on a professional basis have recommended using a single weight, conventional oil, and so far as I know, there have been no problems with using this oil since the engine was first produced in the late 1950s. The FL120 has a reputation for being a 12,000 to 14,000 hour engine in well-maintained recreational service, and they have gone as much as 25,000 hours in continuous commercial service with the Washington State Ferry system according to American Diesel, all of it on the recommended "dinosaur" oil.
So it would seem that there aren't any penalties or even potential penalties for using the oil the engine was designed originally to use. On the other hand, there seem to be plenty of questions about and reccomendations against using synthetic oil (and some engine pros we've talked to even recommend against using multi-viscosity conventional oil in this engine).* So my way of thinking is that even if some of what the pros say is wrong or outdated, why take the risk, however small it may or may not be, when the risk can be eliminated by using what the manufacturer recommended at the outset?
If synthetic is all that's available in some area, so it's a matter of use it or not run the engine, then I guess one has no other choice. But where the recommended single-weight, conventional oil is available, why try to fix something that isn't broken?
-- Edited by Marin on Thursday 27th of May 2010 05:38:11 PM