The Answer
I have now received the answers from the two oil manufacturers regarding using synthetic oil in our old vintage bikes. To begin, the response from the two companies differed somewhat in that one company took the trouble to email me a couple of times before calling me and speaking with me for nearly half an hour. The other sent a more pro-forma email. Synthetic oil and its application is not a cut and dry subject. Obviously, the following is a condensed version of everything I have come to appreciate about the oil products and their application.
There is a big difference between automotive and motorcycle oil. “In 1996 the American Petroleum Institute (API) upgraded the performance standards of automotive oil from SG to SJ (currently SM). This upgrade impacted the friction modifiers and zinc and phosphorus levels, to address the fuel economy, catalytic converter and pollution issues of passenger car owners. For motorcycles, the additional friction modifiers can affect wet clutch performance…”
Motorcycle oils are generally formulated to meet the API SG standard. Motorcycle oils contain higher amounts of wear additives in the form of zinc and phosphorus. This is especially beneficial for flat cam followers. One manufacturer went so far as to say, “API SJ engine oils (automotive type) have a minimal shear stability requirement; therefore, some types may lose their viscosity more quickly when used in a motorcycle, due to the stresses of these bike engines.
There is little doubt that synthetic oil provides better lubrication than petroleum based oil. Synthetic oil creates better “elastohydrodynamic” lubrication –(ain’t my word). This was explained as the oil’s ability to create a full fluid film so that there is no metal to metal contact. At start-up, lubrication (or the lack of it) is critical. Synthetic oil at the lower range has a better “pumpability” than petroleum based oil and so lubrication protection begins sooner.
Synthetic oil is not a uniform product. Some are fully 100% synthetic carry a grade IV rating. Others are what are referred to Hydroisomerized petroleum oils marketed as synthetics under a new law and carry a grade III rating. To further confuse things there are also blended synthetics. In other words, they are a mixture of part petroleum and part synthetic oil.
Let me get to the meat of the matter. The long chain molecules of synthetic oil stand up to wear longer. Shearing and scaring is reduced. The ability to withstand acidification is increased. The Total Base Number (TBN) is a measure of how well oil fights acidification. Synthetics have higher base numbers but not all synthetics are created equal. Obviously acidification is not a good thing but more on this later. While all oil will breakdown over time, synthetic oil withstands this breakdown much better. These issues are especially critical in an air cooled engine. Synthetic oils are more stable over the full heat range. For every 10° C, the reactive rate doubles. Both liquid and air cooled engines run cooler on synthetic. As for wear additives, it is important to remember that these additives are bonded to the base stock hence the same additives combined with an improved base stock translates to improved wear protection.
One area that was of particular interest to me was oil seepage. This issue was described as being “very subjective.” There is nothing in synthetic oil that causes it to seep past gaskets and seals more readily than petroleum based oils on a properly maintained engine as it is somehow thinner or has a lower surface tension. In fact, the viscosity of synthetic oil is more stable over the full heat range. However, if synthetic oil is introduced into an improperly maintained engine including delayed oil changes, acidification has already begun to attack gaskets and seals. Meanwhile the petroleum based oil has begun to “gel” and sludge build-up begins - all of which prevents seepage. Introduce a high quality oil which resists breakdown and voila, a recipe for oil leaks.
If you do not have an oil filter mod and you have not done a rebuild nor cleaned out the sludge trap and you faithfully drop your non-detergent oil after no more than 1000 miles then the benefits to switching to synthetic becomes a less clear. While synthetics provide better lubrication and contain improved wear additives, the introduction of any detergent oil, petroleum or synthetic, could very likely disturb the sludge build-up and as a result may block a critical oil gallery. Without an oil filter there is no way to remove both pre-existing and new contaminants held in suspension. Extending the oil change intervals in these circumstances would (to my mind) be further folly. This leads to a further negative, namely cost.
Many mechanics and experts suggest that oil change intervals should not be extended with synthetic motor oil. Better oil provides better protection within the recommended service cycle. It is argued that extending the change interval defeats this extra protection. Castrol offers a cautionary footnote, “Castrol always recommends that you follow the guidelines of the manufacturer for the GRADE and API specific to your motorcycle’s engine.” On the other hand meanwhile, Amsoil a premium synthetic manufacturer recommends it’s customers they safely can double the recommended factory oil change interval in new motorcycle engines and warranties their product accordingly.
If you really want to know what is going on inside, for $20 you can get an oil analysis emailed back to you within 24 hours of receipt by an independent lab. At the service interval send an oil sample and they'll tell you everything: amount of water, gasket material, boron, sodium, phosphorus, zinc, oxygen, nitrogen, fuel, anti-freeze and the list goes on. In other words; is it safe to extend the oil change interval with a premium product or is it time to consider a engine rebuild.
In summary, you get what you pay for although more reasonable pricing wouldn't hurt. If you’d like your Bonnie to be around another 50 years you have to give it some thought. If you haven’t already had a look, I highly recommend the following informative link:
https://www.amsoil.com/lit/g2156.pdf