I have heard that the polymer chains that give multi-grade its ‘multi-grade’ properties, tend to get chewed up in gears, and presumably the same effect but to a lesser extent could happen with rollers. The oil then will revert back to its mono-grade property of the base stock, which I believe is the lower end of the viscosity range. Quality of the base stock comes into play, oil with a low viscosity index (the rate at which the oil thins as the temperature rises; a low index is bad) will need more polymer additives to reverse this trend and achieve the correct multi-grade properties. Also the lower the oil viscosity range, for example 5W30 oil, the lower the base stock needs to be and the more polymers required to metamorphosis it into heavier weight oil. But it is academic as in a total loss oiling system; you will burn it out the tailpipe long before its multi-grade properties degrade.
I have not heard about the polymers in multi-grade stacking up and impeding the path of the rollers. I am not inclined to believe that, as I do not think they could stack up to become that large an impediment. Ideal running clearance between each roller to race (metal to metal) interface should be 0.00025”. If you had a set of rollers fitted with a smaller clearance than that, such that this polymer stack-up could be an issue, then I think the real problem that leads to bearing failure is that the bearings are just fitted too tight and are overheating. Not whether multi- or mono-grade oil was used. What the polymers are supposed to do is stay curled up when cold, and unravel when hot. I have not yet found any information that they actually ‘link-up’ to form even larger structures. But it would depend if the polymers are measured in the micron, or the sub-micron range; 0.0001” is about 2.5 micron. If they get too hot (localized hot-spot), the polymers are destroyed anyway long before they extrapolate to 3000W treacle! So at worst case the 20W50 oil has the polymers fully unraveled to create 50W, and the engine should after all be able to cope with that.
Also keep in mind rolling element bearings require just a trace amount of lubrication compared to plain bearings. On of the reasons they were predominant in early motorcycle engines with their sporadic and not so reliable oiling systems. You can actually overheat a roller or ball bearing by supplying too much oil, though I think it would be hard to achieve in a motorcycle engine. I saw such a case on a modern CNC lathe spindle that was rated at 4000rpm continuous, but after a few hours operation the main spindle bearings would fail. The water based coolant would turn to steam upon hitting the spindle nose, it got that hot. These bearings were about 4” i.d., class seven precision (minimal running clearance/high precision) and about $1500 each and it used four of them. The bearings had turned a pale blue-grey. After the second set failed (fortunately while still under the service warranty) it was found that the oil drain hole that regulated the oil level in the spindle/gearbox was plugged, causing the bearings to be flooded. As the spindle had some 30hp available, it had no problem cavitating the oil out of the way, but this power churning the oil was absorbed in the form of heat, which eventually started to temper the hardness right out of the bearing. It is hard to say if stacked up polymers in front of the rolling elements will in essence create the same churning effect and lead to excessive heat, or if the heat is coming from just a tight bearing.
The roller bearing element has to roll. If the oil works too well, it will slide rather than roll, possibly breaking through the oil film and allowing momentary metal to metal contact when the loading gets high enough, just before the roller gains traction and starts to roll. In theory then this should make full synthetic oils the worst choice for roller bearing crankshafts, being too ‘slippery.’ People have reported problem breaking in newly re-honed cylinders and piston rings because the synthetic oil was too shear stable. But a search of any number of websites talking about what oil to use in what motorcycle will reveal many posts where oil manufactures recommend their premium synthetic multi-grades for air cooled engines with a roller bearing crankshaft (such as older Urals and air cooled Japanese bikes before they all went to plain bearing cranks), and multiple user experiences to back it up. Another interesting thing to keep in mind is most synthetics have such a high viscosity index they need little or no polymer additives. If there is an issue with polymer stack-up in front of tight fitting rollers, then this would obviously side-step the issue. Good quality mineral base oils also have a high viscosity index, and so need less polymer by volume to do the same job as a ‘cheap’ oil.
However this can all be over-rated blathering. Harley-Davidsons have recommended multi-grade oil for decades, long before synthetics were available, and they have had roller bearing lower ends since time immemorial. But of course one does not pay much attention to the quality of the oil you use in you lawn mower engine either…
Nor have I heard that multi-grades pose any problems for yellow metal bearings. I wonder if that is not a repeat of the old story that the additives in certain hypoid gear oils will attack bronze? EP/Hypoid gear oil first used chlorinated compounds for extreme pressure protection. This was discovered quickly in the 1930s to attack some copper based alloys at moderate temperatures. But not all bronze seemed adversely affected. For example aluminum-bronze as often used for worm gears seems immune. And rather than give up chlorinated EP base oils, manufactures just changed the type of bronze alloy they were using for bearings from the thirties onwards and that seemed to solve the problem. More recently extreme pressure additives are zinc, phosphor, and sulfur based, and these are used not just in gear oils but also in engine oils as well. I have read the sulfur can be a problem with cuprous alloys, but things can be done with it to make it less reactive. I would be very much surprised if the oil manufactures would use a sulfur additive in engine oil that was not compatible with bronze bearing metal. Highly sulfurized metal working fluids (cutting oil) will quickly turn machined copper a dark brown. But at the tip of the tool high pressures exist that break the sulfur down into other compounds that I believe cause chemical reactions with the copper. Chlorinated compounds are still in use despite growing environmental concerns as they are cheaper than the newer alternatives, so beware.
While talking about zinc as an additive, there is a movement here in the US to reduce the amount of zinc used in automotive oils, again for environmental concerns. Now the tappets in the OHV Dougies are quite prone to scuffing and wear, and they need all the help they can get. I worry about them more than the bigends. So if you want lots of zinc, look to one of the oils blended for diesel engines/large trucks. These are currently exempt and still use heavy does of zinc. Or use a synthetic oil such as Mobil1 (multi-grade) or Shell Rotella T mono-grades for diesel engines (Rotella can be a bit confusing, as the name is used on their mineral, synthetic, blended, mono- and multi- grade oils!) which have excellent reputations for very low wear rates, but again are terrible for bedding in piston rings so can burn oil excessively.
As for the subject of viscosity… Traditionally the old total loss engines used 50W, and I have heard occasionally 70W mentioned for the more agricultural designs such as Indian and Harley-Davidson. Basically the higher viscosity was needed due to the primitive seals and oil retention available, and also due to the rather large running clearances mandated by the materials and lack of manufacturing precision. If the original recommendation was 50W, then I do not see why it would not suit today. They were more likely to use their motorcycles in all seasons, where as today our vintage rides are nearly always fair weather events, excepting England where spontaneous rain can never be ruled out. Today we are probably a bit too nervous if we do not see it flowing through the drip-sight fast enough, whereas back then they were stingier with the oil and perhaps oblivious to the consequences till after the first seizure. But then they were not running motorway speeds, where a momentary loss of oil supply precedes seizure by mere seconds. Also back then there were a network of Douglas agents available to order replacement parts from! Probably in a total loss system anything from 50W down to perhaps 30W would work just fine, with 30W probably smoking quite a bit more with used cylinders on warm-up (if such environmental responsibility concerns you.) If you feel that on the low pressure delivery side the oil is not flowing freely enough for you climate, by all means drop down one viscosity range, or use a multi-grade. Avoid low viscosities, as even if you have upgraded the sealing arrangements, and re-machined all the wear surfaces, the primitive design and air-cooled nature dictates that you are not going to successfully employ modern thermally analyzed, water-cooled engine type clearances. These multi-weights that go down to 5W are just to reduce cold weather starting effort, primarily through reduced drag at the piston skirt, and allow quick pumping up of the oil into the minuscule clearances now employed. On a Dougie they will do as an emergency supply if you run out of your favorite, but just find their way past the piston rings and into the atmosphere all that much quicker, polymers notwithstanding! With larger clearances, you want a more viscous oil.
It sounds like from your description the OC has an oiling arrangement very much like the early EW. Douglas abandoned that on the EW fairly quickly; it is a mystery why they did not do the same on the OC. As you describe, it has to draw the oil under vacuum out of the oil tank compartment, up through the sight glass, and then it can run down to the engine by gravity. The slightest air leak in the sight glass or at any fitting on the suction side will render the system useless, and it is more difficult to make a system air tight than it is fluid tight. Probably an air leak is your culprit. But if you think the oil is too viscous to flow through the system, then you could try a multi-grade as you proposed. Until it gets to the engine, the oil is relatively ‘cold’ so at the lower viscosity. The problem is the only 50W multi grade I know that is readily available is 20W50, and I think the 20W is a little thinner than I would care to run. Once in the engine (once it was warmed up) the viscosity ought to increase, so perhaps it would work just fine and it is easy enough to try a quart. Of course, ideally you do not want the viscosity to change at all. If the engine is designed to operate with 50W, you want the oil to behave like 50W inside the engine at all times; when stone cold to stinking hot.
Just a few thoughts.
-Doug
