I there any reason why switching to synthetic oil would cause an engine to run hotter? Recently, I switched from Quaker State 15W40 to Mobile 1 5W30 synthetic oil and the engine runs hotter. Could the weight change have caused the engine to run hotter? I suppose it is possible that something else is going on with the engine.

 

You went to a thinner oil?!?! For what, 5 horses?!?! Oil is the only coolant for the bottom end of the engine. And warmer weather will be here soon.

 

When I had a new engine installed 55,000 km ago my mechanic suggested using 15W40 oil which has been changed regularily at 5000km intervals. When I switched to synthetic I was advised to use what the manufacturer recommended which is 5W30.

 

Most likely has nothing to do with the oil.

 

The over heating problem was finally diagnosed and solved. It of course had nothing to do with the engine oil, it just happened to occur at the same time. I guess that is what you call a co-incident.

Any how after checking everything I noticed the over fill tank was not rising and falling as the engine heated up. So after the engine cooled I removed coolant from the radiator to well below the core. Then the car ran cool. I concluded the radiator cap was defective. A new cap corrected the problem.

 

I've heard of synthetic oil being blamed for flat tires.

Glad you found the problem.

 

Im still a conventional guy when it comes to flat tappet motors.

And, I have grenaded a engine after a synthetic oil swap, but the 120degree
heat and ignorance had nothing to do with that one whatsoever.

 

I hate to turn this into a "my dad's bigger than your dad" thread, but since you brought up flat tappets, I will respond.

"Conventional" oil does not protect flat tappets better than synthetic. In fact, it is worse. The issue revolves around ZDDP anti-wear additives - with the advent of the ILSAC GF-4 standard, the max allowed was reduced for the sake of emissions-control equipment (beyond 3rd gen, BTW). Since almost all new vehicles have roller lifters, that isn't a problem. But, for a flat tappet engine, the worst case scenario is a petroleum-based oil rated ILSAC GF-4. The answer is to use an oil that isn't ILSAC GF-4 rated; a simple choice is a diesel oil that is also rated for gasoline engines. As you might expect, a synthetic diesel oil rated CI-4+/SL is going to protect your lifters better than a CI-4+/SL petroleum-based oil - both will have the ZDDP anti-wear additives in sufficient quantity to protect the flat tappets, but the synthetic (especially Group 4) will always have the upper hand in this and overall lubrication protection.

I only went off-topic to address an off-topic post, so please, don't turn this into a synthetic vs. petroleum oil debate.

 

are you serious? the question was clearly resolved..
my humor was not meant to strike a debate or anything!

You could have supplied that morsel of information without
putting me in your cross hairs or ripping my post up.

 

what type of oil you running?