The article below is about gear lubrication, which is similar to some components that we might be hearing, although the oils being used are very different. (except for some thick oil fans that just put a zerk fitting on their oil filler cap and add grease as needed to their engine :^)
What's interesting is the discussion about film thickness vs temperature, where based upon complaints about noise using synthetics it seems that most pronounced observations are with cold engine, even if it's still noiser when warm ? A reasonable guess is that synthetics are producing a thinner oil film thickness at lower temps, and thus there is more clatter in some engines. Obviously viscosity is an issue, as M1 5w40 is pretty quiet in either Taurus, as is M1 0w40.
http://www.machinerylubrication.com/article_detail.asp?articleid=586&relatedbookgroup=Lubrication
Selecting Oils with High Pressure-Viscosity Coefficient - Increase Bearing Life by More Than Four Times
In the following article the author explores the ageless question: Are synthetic oils (PAO - Polyalpha olefin and PAG - Polyalkylene glycol) better gear and bearing lubricants than mineral oils?
Practical Guidance for Selecting Lubricant
(as demonstrated by Figure 3):
PAG oils have significantly thicker films than PAO and mineral oils over the entire range of practical temperature.
There is little difference between PAO and mineral oils for the temperature range of 70°C to 90°C. In this same temperature range, PAG lubricant gives 16 percent to 37 percent thicker films than mineral oil.
For temperatures less than 70°C, mineral and PAG oils have significantly greater film thickness than PAO oils.
For temperatures greater than 90°C, PAO and PAG oils have significantly greater film thickness than mineral oils.
