Originally Posted By: turnbowm
Originally Posted By: BrocLuno
I have been saying for a l-o-n-g time that high quality dino oils are good.
And I have been saying for the same amount of time that synthetics have issues. I'm not saying don't use them. I might as well be whistling in the wind to take that stand. But, I am saying be careful in your selection ...
What are the issues with synthetic oils?
Have to be careful here as some synthetics like ester based ones don't exhibit the same issues, but ... Some synthetics exhibit drain-off. You can google that and get all sorts of stuff to read. That means that the residual film is still there, but very thin. So when you fire the engine, it'll clatter and click for quit a while. That's the general downside.
Not all engines will do this, just some of them. So when you hear this, you might consider switching oils as you are hearing moving parts with some separation with little oil cushion. It's the same nature of the liquid that allows very low pour points and wide viscosity ranges like 0W-40 w/o resorting to a bunch of VII additives.
The other issue is capillary fill. It's a measurable phenomena. A good Group II+ will exhibit good capillary fill. Most synthetics will not. This is important for big end bearings. If you hear a low pitched knock on cold start, your big end rod bearings have little cushion. They'll fill after a while with pumped oil. But those first say 500 rpm, they are running pretty loose and semi-dry. If you are a drive -off kinda person, you can be loading these bearings w/o a decent oil cushion.
New motors are tight. But those of us who build loose engines or drive vehicles with 200,000+ miles on them know all about this stuff. So you choose an oil that will help you work around these issues. Most times a semi-syn will do it. Sometimes it's best just to go straight dino oil. Let your ears be a diagnostic tool. Listen to cold starts with the hood up and see what you hear ... Act accordingly.
On the plus side, the high NOAK that they are getting worked up about above means little if you are changing oil every 4K. You have not flashed off that much.
Since marine engines tend to run cooler than auto engines, they are often fed dino oil and run for 5~10,000 hours quite happily. Ships engines may run 20,000 hours before any serious work, all on dino oil. Sump temps are low(er) so NOAK is not such an issue.
So one has to examine the operating envelope of the engine at hand. Coolant temps of 205*F (norm) and sump temps of 260*F or so would tell you that a synthetic would be best, especially if a turbo is involved. But a farm tractor with a 175* thermostat and 4 gallons of sump oil will be quite happy with dino. Most stuff is somewhere in between
