Yes, motor oil. The evidence is pretty strong that we are dealing with water in that front axle housing. My judgement is that getting every drop of water out of those convoluted bevel-gear housings is a whole lot more important than what oil we end up running in that axle housing. After all, if you look at old factory literature, Yanmar themselves never could decide which oil should be used.
Motor oil will do fine. It has the detergency to help form an emulsion to hold any remaining water in suspension until we can drain that out, too. As for the multiweight aspect, that's an advantage we also get with motor oil, and it's a big advantage if we are trying to deal with changing axle oil this cold winter. For you tropic birds, it's winter here in the non-california USofA. When the temperature drops into single digits, GL90 just doesn't flow well enough in the cold to even get it reliably into or out of the axle housing. Then try to figure out where the level should be when the GL90 has roughly the viscosity of honey. Add a sprinkling of wind-blown snow, and motor oil is easy to deal with by comparison.
We mentioned on this forum before that the whole subject of what lube for the 4wd front axle never was never really resolved by Yanmar. Reading through twenty year's worth of their technical publications, they are quite proud of having invented that type of 4wd front axle, but in their shop manuals they quite often recommend two completely different oils: using either a light oil and a heavy gear lube in the front axle ---and they do so without any word of explanation as to which to use or when. The only way that makes sense is that they thought chosing one over the other was simply common sense. So I'm sort of agreeing with the Yanmar designers. I personally think that those guys got tired of debating which oil to use in their new fancy axle and just ended up solving the problem by specifying two completely different oils. You don't have to be a lubrication engineer to see the problem: their nifty axle has lots of sliding bevel gear friction (GL lube indicated), and some ball and roller bearings (light oil recommended). The axle assembly also has some seals which operate "wet" continuously. Those seals will benefit from some of the motor oil additives & the combustion additives won't matter. There are good arguments in favor of a multiweight high detergent oil with additives there - and motor oil is an easy way to get it.
Bottom line is that a real front axle failure is almost unknown in Yanmars as long as the axle has any kind of lube in there at all (although leaky seals are common enough). Just get the water out - that's the important thing. Consider changing it several times.
End of rant. And btw, California....thanks for asking question.
rScotty
Just finished draining an refilling the front axle oil (about 3.5 qts on the dip stick indicator) reservoir with 10-30 motor oil with detergent. The oil that drained out was not 90 wt gear oil best guess 30wt oil. Outside temps are above freezing and the steering was not frozen. Checked out the Yanmar service manual that called for 90 wt gear oil. All in all rScotty thanks for the education I'll run this oil for a while then flush again with motor oil as oil is cheap so far and it gets recycled in the oil/wood stove to heat the house. Thanks for the input.
