Viscosity and hours between oil changes in small air cooled engines.

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I too have mulled what weight oil to use in my generator and other OPE. I've come to the conclusion that if it's a splash lubed system to use a 5w-30 just because I want the oil quickly flowing down the block to the sump so it can get splashed around again. I don't want a 40 or 50 weight taking more time getting back to the sump to get dispersed again. It's doing no good hanging out of the walls of the block while on it's way down. The main thing is the small amount of oil they often hold. 16 ounces isn't much if it's not getting back to the bottom of the block. To me, a constant flow of thin oil is better than no or a small amount of a thicker oil any day. And since the system isn't pressurized, seems to me that a thinner oil will get into tight areas better than a thicker oil will. Changed every day in extreme conditions should keep it fresh enough to protect the engine. And, I don't trust the low oil cutoff either because I think there would be enough residual oil on the switch to keep it from shutting the ignition off and not necessarily enough to lube the engine. I use Pennzoil Ultra Platinum 5w-30 in all my OPE. Pressure washer, snow blower, push mower, and generator.

As far as pressurized systems, I run Mobil 5w-40. I like the idea of the slightly thicker oil in an air cooled engine but the splash lubed systems just don't hold enough for my liking. And even being filtered and pressurized, I usually run 50-75 hours on a recommended 100 hour change. It's cheap and I change the filter every other time...the manual recommends 200 hours on the filter BTW. On a hot day, I have used my infrared heat gun to measure the oil temperature and have seen as high as 265 degrees for hours on end.
 
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When the oil is moderately cold there’s a lot less difference in viscosity that’s you’re imagining between a 30 and a 40-grade, even a 50-grade. When it’s hot that will make zero difference in drain time. And “tight areas” is irrelevant.

Thin oils are never better at protection MOFT always wins.
 
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How would tight areas be irrelevant? If a higher viscosity oil can't get in between, say, the piston and wrist pin area and the thinner oil can, then there is going to be metal to metal contact.
 
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How would tight areas be irrelevant? If a higher viscosity oil can't get in between, say, the piston and wrist pin area and the thinner oil can, then there is going to be metal to metal contact.
So you’re saying you have some sort of technical proof that this doesn’t occur with say a 40-grade as opposed to a 30-grade? Something other than your feeling that it does not. That the oil molecules are too big to fit.
 
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No technical proof other than if higher viscosity always wins then why doesn't every engine just get 100 weight and call it a day. Obviously there is a need for thinner viscosities for tighter tolerances to get lubricated where a higher viscosity cannot physically get into. What I'm saying is that in a non-pressurized oil system, you are relying on oil to get into tight areas via gravity or just parts moving and oil flying around and that's why I prefer a lighter weight oil. I totally agree that a higher viscosity oil is better at keeping metal components apart from each other as long as the heavier oil can get in there. And maybe a 40 or 50 weight oil at 250 degrees can get in there but I'm just not willing to try and like I said earlier, I want that oil to get off the part and back to the sump to get kicked back up by the dipper on the connecting rod to lubricate parts again.
 
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I too have mulled what weight oil to use in my generator and other OPE. I've come to the conclusion that if it's a splash lubed system to use a 5w-30 just because I want the oil quickly flowing down the block to the sump so it can get splashed around again. I don't want a 40 or 50 weight taking more time getting back to the sump to get dispersed again. It's doing no good hanging out of the walls of the block while on it's way down. The main thing is the small amount of oil they often hold. 16 ounces isn't much if it's not getting back to the bottom of the block. To me, a constant flow of thin oil is better than no or a small amount of a thicker oil any day. And since the system isn't pressurized, seems to me that a thinner oil will get into tight areas better than a thicker oil will. Changed every day in extreme conditions should keep it fresh enough to protect the engine. And, I don't trust the low oil cutoff either because I think there would be enough residual oil on the switch to keep it from shutting the ignition off and not necessarily enough to lube the engine. I use Pennzoil Ultra Platinum 5w-30 in all my OPE. Pressure washer, snow blower, push mower, and generator.

As far as pressurized systems, I run Mobil 5w-40. I like the idea of the slightly thicker oil in an air cooled engine but the splash lubed systems just don't hold enough for my liking. And even being filtered and pressurized, I usually run 50-75 hours on a recommended 100 hour change. It's cheap and I change the filter every other time...the manual recommends 200 hours on the filter BTW. On a hot day, I have used my infrared heat gun to measure the oil temperature and have seen as high as 265 degrees for hours on end.
You are over thinking this.

The most important factor in a splash lubed system is that the oil level is full. I've worked on small engines for customers going on 16 years now. I've had customers bring me 30+ year old machines with the original oil in them that they have never changed, only added when it was low. The machines might smoke a bit on start-up, but otherwise run fine. Small engines have very loose tolerances, and very little complexity inside. Oil getting into tight spaces isn't an issue like it is on modern automotive applications with hydraulic controlled variable valve timing. You could run anything from a 0w20 to 10w60 and they will be fine as long as there is enough of it in there to get splashed around. Every single engine failure I've seen that was lubrication related the engine had either no oil at all, or had been running very low on oil for quite some time.

My advice to anyone looking for generator oil is pick something that fits your maintenance schedule. If you check the oil every time you fill the fuel tank and plan on changing it every day, then basically anything that is oil will work, preferably something in the viscosity recommended by the manufacturer. If you rarely check the oil, run the generator for multiple tanks of fuel unattended, or work in extreme heat, then a HDEO might be a better choice just because they are less likely to be consumed.
 
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Plus if a 40 or even a 50-grade oil couldn't get into tight spaces or was really a problem in cold weather then my 21 year-old snow thrower would be in the scrap heap by now. Sometimes I really wonder how large people think that an oil molecule can be.

The notion that I'm advocating a 100 "weight" oil is just silly and isn't germane to this discussion.
 
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I have a +50 year old B&S MontgomeryWard 5HP 2200 Watt generator that I use HD 30 Wt oil in. We used it for 2 weeks in 2009 when we here in Western Kentucky had a massive ice storm. We have 2 homes so it was taken from house to house and boosted freezers for co-workers. Oil was changed once. Still runs good. Put in back to sleep till the next big event. My only complain is that the 3 quart gas tank runs out after 45 minutes.
 
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I run 10w30 Full Syn. QS in all my small engine equipment with non ethanol fuel and stabil in every tank. Never had any engine issues and my equipment is always ready to go.
 
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