924Board.org

[emoore924]

Ok, so I was changing the oil in my Motorhome's Onan 4.0 genset this past weekend. It is supposed to take 3.5qt of straight 30w. I drained the oil but then realized that I only had 3qt of 30w, not 3.5. So I poked about the garage and found a 1/2 qt of 20w50 Brad Penn leftovers, and figured wth, its a genset, how picky could it be.

I replaced the filter, then dumped in 3qt of carquest 30w and 1/2 qt of Brad Penn 20w-50, and fired it up. Sounded fine, ran fine. HOWEVER, when I pulled the little dipstick to check that the oil level was right, I noticed what seemed to be quite a bit of foam in the oil, but that's just an impression. I have no idea if this was a lot or a little bit of foam.

Now one normally wouldn't see into the pan when you're checking the oil on a car engine of course but that's what you're actually can see in the case of a genset -- the oil in the pan. It reminds me of foam on a beer after its been sitting for 20 min or so -- small bubbles clustered around the edge of the glass so to speak, but it would be the edge of the oilpan in this case...

So, is this:

1) Normal, fughettaboudit
2) A result of the mix of 30w and BP? and is it a probem, or not?
3) OMG, quick, change the oil...??

I just don't have a feel for the anti-foaming characteristics of these oils and whether this is something I need to worry about. I know foam is not the best lubricant. Add in that the the genset (powers the coach and a/c) sees severe duty at the track, so it has to work and it has to be right or I get very hot in the dark....

[gerben1986]

Problem with your headgasket maybe?
What was the colour of the foam?

[musicalannette]

With oils there are two viscosity raitings. One is the viscosity (or how thick) the oil is at 0 deg C and the other is the viscosity rating at 99 deg C.

For Example ;

0W30 has thin oil at 0 deg C and medium/light thickness at 99 deg C
20W50 is thick at 0 deg C and thick at 99 deg C
0W thinest, 10W thicker 20W thicker still at 0 deg C
W30 thinest W40 thicker W50 thicker still at 99 degC

the ideal oil is 0W50, thin at startup, thick when its hot. (unless you go racing or are being really anal about internal drag of the engine).

If you mixed them and they were semi or full synthetic with mineral oil, they are likely to globule or emulsify and not mix fully, however if they are both mineral based they should mix together ok, you expect a little frothing from passing through gear driven pumps, but by the time the oil has been through the engine and drained back into the sump it is normally not airated unless you have a dry sump setup and it has passed through a scavange pump on it's return. Some ONAN gensets use motorcycle engines for weight reduction, you might want to check this out as it maybee normal. Also you might want to check it's not just on the surface of the returned hot oil, it might not be the bulk of it, remember the pick up pipe is submerged in the oil, it might be fine down there.

But at the end of the day, if in doubt - oils cheap, the bearings, camshafts, liners are not and then there's the time issue, 20 minutes for an oils change is quicker than a day or two for a stripdown.

Not much help really i know, it should be ok, just have a check there isn't a scavange pump in the circuit or its just on the surface, this might explain a whole lot. If not, then just change the oil, better safe than sorry.
_________________
I KNEW white wall tyres were invented by Americans .....just not at Boeing.... to be fitted on the 737.....

[MikeJinCO]

I have the 2.5 ONAN in my camper. I don't particularly like them as they are heavy, noisy, and expensive, plus I've heard that any repairs are extravagantly expensive. but I can start it up from inside with its electric starter for morning coffee. Mine has a horizontal cylinder without a sight glass, the entire unit is only 8" tall. If the 4.0 has horizontal cylinder(s) I would suspect that any foaming is caused by the crank moving thru the oil in the sump. There is probably some foaming anyway as straight 30 wt is not going to have any of the new anti-foaming agents. One part in seven(1/2 qt to 3-1/2qt) is about 14% different oil will mix right in and the motor probably won't know the difference. Just my opinion.
_________________
Mike


'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)