Roger,
Sorry for the intrusion, but motor oils has been kind of a hobby for me, so I hope you don't mind my opinions. You probably already know this. The first number in the oil weight relates to cold flow. Usually a 20 is perfectly fine down to freezing. The second number relates to viscosity when hot. Oils with a wide range, 5w-50 for example tend to shear down more quickly than narrower range oils like 20w-50. Castrol 5w-50 shears down very quickly. Since this engine has a separate gearbox, this will slow the shearing a little. Shearing is when the oil effectively becomes a lighter weigh oil because the long chain molecules break.
Synthetic oils have a more consistent molecule length. Conventional oils vary in length, with long and short and are blended to achieve an average length. Long molecules break down more quickly than short ones. Which is a reason conventional oils tend to be less durable, there are longer molecules, but also fewer of them.
ZDDP provides a very thin barrier between a steel part and the other surface. If the surfaces (rod end bearing for example) penetrate the oil film, the ZDDP provides a small amount of protection but not as much as most people think. An engine with proper bearing clearances and oil pressure will very rarely make metal to metal contact.
Moly is another barrier of protection. Acting like slippery plates that stack up on steel surfaces and slide off when metal to metal contact is made. Moly tends to be more effective, but has been shown to reduce ring life in some engines.
Oil companies have new proprietary additives which don't show up on an oil analysis. ZDDP is an old school trick, but has been reduced in car oils due to environmental concerns.
HDEO (heavy duty engine oils) tend to have higher levels of ZDDP, as do motorcycle oils. European spec oils also tend to do quite well, and can not only be identified by their weight but have VW, Porsch, BMW, or Mercedes approval. They have demanding requirements for certification.
Viscosity will vary by brand. I have seen oil analysis of 40 weight oil that was thicker than 50 weight oil.
If you want an inexpensive oil with good wear numbers Rotella is hard to beat. Mobil 1 15w-50 has good levels of ZDDP and has been a motorcycle favorite for years. Amsoil buys their PAO base stock from Mobil, but has more "synthetic" than Mobil does and outperforms Mobil. I personally prefer Amsoil products. The high mileage oils have more ZDDP and work well with high revving engines, but you might not find the weight you are looking for. Redline is ester based, holds viscosity very well, but sadly gives bad wear numbers, and Lucas oil is just horrible. I'm not familiar with Polaris oil, but most manufacturer branded oil tends to be mediocre. For example Honda motorcycle oil is pretty crappy, but they make good engines.
Oil that tends to quiet rattles in engines is due more to their viscosity than additives. A microscopic coating of ZDDP isn't thick enough to acoustically dampen the sound of slapping metal parts.
According to Blackstone Labs (oil analysis company) wear numbers between different oils tend to be quite small. A great oil doesn't do that much better than an average oil, and typically only has a few less PPM of wear metals. Modern oil in general is quite good, the key being to properly match the oil to the application. I don't use the new car oils in anything I own. I use euro spec, motorcycle, or HDEO oils. In my Jabiru I use Exxon Mobil Elite semi synthetic 20w-50 aviation oil and Blackstone tells me I have some of the lowest wear numbers they have seen, so I'm sticking with it.
If you aren't bored to death by now, and want to get much more useful information on oil I highly recommend bobistheoilguy.com
Sorry for the long post,
Kevin K.
Sonex Jabiru 3300
