Replaced conventional oil with full synthetic.

[Ronnie]

I've heard a lot of people say if you change oil and use full synthetic in an old car that's always had conventional oil in it that it will be more prone to leaking. I thought I would test out that theory on my old GMC pickup with 150k miles on it. I'm a Castrol GTX man who has used it for over 40-50 years in all of my cars but it never hurts to try something new every once in a while. 🙂 

 

On the last oil change I decided to try Castrol Edge 5w30 full synthetic in the old truck. It has always had a few drips under the engine since I've owned it so this was a good test to see how full synthetic effects a leaking engine. 

 

With the full synthetic it leaks twice as bad, or maybe worse than that. I can tell by how much is on the new piece of cardboard I put under it when I changed the oil. I usually change oil in it every 3000 miles in my truck but it's leaking bad enough that I'm going back to Castrol GTX after only 1200 miles since changing to full synthetic -  which is supposed to be good for 10,000 miles.

 

I can tell the oil pressure is slightly lower too but that might be due to the unusually warm summer we've had.

 

My Equinox and my daughter's Camry don't leak a drop with Castrol Edge. My Mercedes doesn't leak a drop with with full synthetic either. They were designed for full synthetic and that's all they've ever had. For some reason my 2001 GMC pickup that's always had Castrol GTX conventional oil doesn't like full synthetic so I'm going to stick with GTX in my old cars from now on. 

[TripleC321]

Strange. I run full synthetic in my Reatta and it doesn’t leak anymore than a 35 year old engine would. 
 

which at least for me right now, is a slight oil pan leak. Very small. 

[ship]

Ronnie, 5W is pretty thin oil when it cools or is cold.  Have you always used 5W in your truck?  Maybe 10W30 might be better.  I've always use synthetic 10W30 in all my older Buicks.  My current '91 Reatta with 110K miles, my previous '92 Riviera with 165K miles, and my previous '85 Riviera with 125K miles started out with conventional oil.  I changed them to synthetic 10W30, and they never have/had any dripping oil leaks.

[Ronnie]

I think the truck is leaking from the rear main seal. Maybe the full synthetic made it leak worse because the crankshaft is spinning and slinging the oil out when the engine is running versus it just getting past a gasket. I will be changing back to the GTX soon and if the oil leak goes back to the normal dripping I will say the full synthetic made the leak worse. If not I will say the leaking getting worse was just a coincidence. I really need to fix the leak but I just keep putting off having it done.

 

 

[DAVES89]

My wife's car gets full synthetic as the owners manual requires it. My two Reattas get a synthetic blend. I run 10w30 and change at 5000 miles or so.

[2seater]

Maybe one of the high mileage oils would help? I think they have a seal additive to soften and perhaps swell?

My personal experience has been that it may worsen an existing leak but not cause an otherwise dry engine to start. Tribal knowledge seems to indicate the synthetic has greater wicking ability? It seems strange there would be large differences between tue two since they can be blended or used in combination?? Two of mine require 0w-20, so no choice, but I do run 0w-30 in my other two that are specd for 5w-30. One does drool a little but it was a seeper before and my Buick is dry as can be.

[ship]

My Generac generator takes 5W20.  As I get older, I find it difficult to see the oil level on the dipstick when it's at 5W....it's real thin stuff.  Can't imagine trying to find 0W on a dipstick!

[Ronnie]

8 hours ago, 2seater said:

Tribal knowledge seems to indicate the synthetic has greater wicking ability?

I've heard that to. Maybe that is skewing my highly scientific test results. The spots the full synthetic left on the cardboard seem to be about twice as big as they were with the Castrol GTX conventional oil on the old cardboard. That might possibly be caused by the composition of the new cardboard being more absorbent than the old cardboard, and/or the full synthetic oil having a greater wicking property. 

 

I think more testing is in order before draining the full synthetic and going back to conventional oil. I'm going to jack the truck up and visually inspect the bottom of the engine to see how bad it appears to be leaking. Then I'm going to try a better grade of cardboard to see if the size of the spot changes when oil drips on it. Does anyone know if the Rockwell scale can be used to test cardboard to see which one is harder and less resistant to oil spots?   🤣

[Ron Walker]

Gotta love that cardboard - 101 uses!

[DAVES89]

Was getting a low oil level warning on the Red which usually means time to change the oil which I did. But still getting the warning so I think it's time to change the sensor. Second time in 322,xxx miles...