Thanks for the replies. The guy isn't a mechanic, just a guy who has brought up some rather strange points about synthetic oil use. For instance -
(BEGIN QUOTE)
I personally am not a fan of synthetic oils in an internal combustion engine for several reasons;
1) Usually costs 2 to 3 times as much as conventional oil.
2) The manufacturer recommendation.
3) Synthetic does not burn well.
Burn well? you ask. Every stroke of an internal combustion engine leaves a light film of oil in the combustion chamber. With dino oil, this film is combusted. With synthetic, because it has "such great thermal properties", the oil glazes to the combustion chamber and forms a ridge over time. This ridge will reduce the combustion area in effect raising compression. Over time, the compression can become to high resulting in combustion gases blowing by the piston and rings into the crankcase. The increased pressure will eventually take its toll on the connecting rods, bearings, and crank resulting in premature engine failure and more frequent engine overhaul.
Big diesel engines are harder on motor oil than any internal combustion engine. Do some research and fine out what kind of oil those big diesel engines are running. Synthetic was developed for gas turbine engines, not internal combustion engines.
Most conventional motor oils have additives that include a synthetic component to improve thermal properties. They are cheaper than pure synth and provide adequate protection.
Synthetic oils have been around for decades. If they were really as good as their manufacturers claim, why do you think that most automotive and motorcycle manufacturers use conventional oil from the factory?
I know there are a lot of proponents of synthetic oil on this forum. I don't mean to start a war, but would recommend that you do your homework before deciding to make the shift to synthetic in an internal combustion engine. (END QUOTE)
I guess "they" have moved on about flat spots on bearings, and engines not breaking in. And I never knew that diesel engines weren't internal combustion engines!