This is a hard subject to discuss because everyone has an opinion based on what the dad grandfather, mechanic or friend has told them, or based on some of the conflicting data which is available on the Internet, some magazine, or book. The bottom line is: everyone should follow what they think is right.
Here is why I do what I do Based on almost 40 years of working on and maintaining my own vehicles.
Here is why I believe thicker oil is better for a engine, especially a small aluminum engine like the ones we drive. Remember these are only my opinions.
Myth 1, Thicker oil takes longer to get to the Bearings; The oil moves from the oil pan through the oil pump and is pushed under pressure through the oil galleys to the bearings where it is pushed past the bearings and makes it way back to the oil pan, to begin the process again. This is ideally a modified hydraulic system. The oil pump pushes all oil at the same speed it can not push it slower unless the resistance is high enough to break the pump. It used to be on old V8's that the oil pump was run off the bottom of the distributor gear, and sometimes with that design the pin which holds the distributor gear to the distributor would sheer if the Dino oil was to thick. This is no longer an issue at least on our little engine as it is direct drive off the crank shaft. I have been running 20W50 Syn in my lil Suzy since 91. and at least for 150 of her 170,000 mile life. I have never had any problems no matter what temp it has been outside.
Further the idea is not to pump the oil to the bearings faster. The idea is to leave the oil suspended in the galleys and on the bearings and cylinder walls until restart. This way the oil doesn't have to get anywhere, it is already there when the pump starts. This is why I use thicker oil as it runs off the parts slower than thinner oil. It is like lubricating a hinge with WD40 or 3in1 The WD40 hinge is going to need to be re-lubed much sooner than the 3in1 because it is a thinner oil. I should note at this point that I have seen the oil light stay on for a moment or two on some vehicles I have started which were using thin oil. I want you to know that I never see the oil light on lil Suzy at start up The only time I see the light is if the engine is off and the ignition is on. As soon as the engine cranks there is at least enough pressure in the system to kill the idiot light. I do not have a pressure gauge in lil Suzy so I do not know what the actual pressure is.
Another factor is the oil rings Many people think that the oil rings keep oil out of the cyl. This is not the case, the compression rings actually do a much better job of scrubbing the cyl. walls. The oil ring is designed to hold oil and lubricate the cyl. as the piston moves up and down. The ring looks like this- lVVl - the two thin rings on the top and bottom allow oil into the middle where it is held by a ring of zigzag spring steel which holds the outer rings apart and also creates small spaces which hold oil in reserve to keep the cyl walls continuously lubed. and to hold oil until the engine is restarted. This is another reason I use thicker oil, again because the thinner oils will leak out and return to the oil pan sooner, leaving the cyl. walls without lubrication.
I reread the 95 owners manual I referenced above and discovered I was misreading the oil page. Suzuki gives the temp range for 20W50 as good to -10 degrees C. or 14 degrees F. I still don't see this as a problem due to the superior cold weather flow characteristics of synthetics. On the other end 10W30 is only recommended up to 86 degrees F. I have given some thought to why Suzuki would allow such a wide range of oil viscosity's to be used in their engines. It would seem that if there was a optimal engine oil for these engines They would list maybe 3, 5W30 for really cold climates 10W30 for medium and maybe 10W40 for hot areas. Why would they add the heavier oils? The conclusion I have come to is: It depends on how hard you are working the engine. Thicker oils have higher sheer strengths, that is, how much pressure the oil will withstand before it separates and allows metal to metal contact.
I have used lil Suzy quite hard over the years. When I was mud bogging her in Memphis Tennessee along the Wolf river, I can not count the number of times the radiator got plugged with mud and I drove her to the closest coin car wash with the engine temp gauge in or close to the red zone. Even with all that and 170,000 miles she still gives good compression numbers and runs quite well given her age. I still work her quite hard when she is on the trail, partly because I have not gotten her new T case gears installed yet, She is asked to climb steep hills and crawl over big rocks at very low rpms which she does quite well and further, she does not burn any oil to speak of. The plugs look fine.
As I mentioned before there is a trade off in mileage due to the fact that thicker oil is harder to move things in. I am not sure how much of this is off set by the increased lubricity of synthetics but I am sure there is some reduction.
Again I want to stress these are only my ideas and I am not recommending them to anyone unless you think they have Merritt.
Dave
