Originally Posted by GreatWhiteNorth
These articles regarding oil life mention how quickly oil breaks down changing viscosity. A lower viscosity wouldn't provide as much protection to delicate plain bearing surfaces, right? Am thinking this may factor in to galling often found in the cam bearing caps? I sure notice how much thinner my oil is when I drain it after 2000 miles as compared to when new. I've been using nothing but Castrol GTX 10w40 mc oil, but have started recently adding Shell Rotella T synthetic (HD diesel oil) when topping it up to restore viscosity as the oil wears. ...not as good as Mobil 1, but a pretty good oil IMO.
Lots of mis-info regarding viscosity...Here is a post I did over at another KLR forum. Hope that helps explain it better.
While at a HD function for females I was off with the guys outside and they were all talking bikes and oil topic came up. I kept quiet as long as I could but finally after hearing this one particular guy rant on and on about oil weight, labels and what to use in his HD during summer/winter as well as the whole synthetic vs dino I couldnt contain myself as he was completely wrong even on the most basic of all oil information. That being what does 10W-40 mean and what about oils that are rated SAE 30? Should you run 20W-50 in summer?
I quickly realized that not one person even knew what the W meant. That said I thought I'd post up what I explained to them. Turned into quite the conversation and quite a few Ah ha's from other riders there.
Many falsely believe the W means Weight....it doesn't. Oil weight, or viscosity, refers to how thick or thin the oil is and labeled by number(s). The temperature requirements set for oil by the Society of Automotive Engineers (SAE) is 0 degrees F (low) and 210 degrees F (high).
Oils meeting the SAE's low temperature requirements have a "W" after the viscosity rating (example: 10W), and oils that meet the high ratings have no letter (example SAE 30). The "W" designation means Winter. Again, the temp ratings are for oil temps not ambient temps.
The 40 in a 10w-40 simply means that the oil must fall within certain viscosity limits at 210 degF. This is a fixed limit and all oils that end in 40 must achieve these limits. Once again the lower the number the thinner the oil, a 30 oil is thinner than a 40 oil at 210 degF etc. It does not mean a 50 performs better at higher temps it means its viscosity is rated at 50 at 210 degF. Big difference....
So take the 10W-40 example...Already explained the 40 but what about the 10W? Simple...it simply confirms the oil has viscosity rating of 10 at the SAE temp of 0 degF. If there is no W designate it simply means the oil has viscosity characteristics of 10 thru 40 and meets the 40 rating at 210 degF.
An oil rated at 30 and a similar oil rated at 50 will both begin to 'burn' at the same temps. Ester and Synthetics will withstand higher oil temps before they begin to break down (burn) but viscosity ratings (the numbers) simply measure just that at 210degF....viscosity.
As others stated you should purchase/use the oil as outlined in your owners manual. Different engines (bearings actually) require different viscosity. My Viper requires 0W even in our 115F summer heat.