Question on oil

Question on oil

justinkase

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Location
Laramie, Wy
Vehicle(s)
1985 cj7, 4.2L I6 w/.06 overbore, T-5, dana 300, amc 20, dana 30, 4.10 gears
I changed the oil in my new to me 85' CJ7 for the first time the other day and the guy at the parts store told me that since the last owner used synthetic that I should to, or it could cause leaks to go back to conventional. Is this true or did I get took for a ride.

Also, I took it out today to go play and on the way home I noticed that I have a vibration that starts at about 30 in 3rd gear, but goes away in 4th. Could this be a u-joint going bad?
Have not learned these things yet and some help would be great.
Thanks.
 
I've used both Mobil 1 and Rotella T in my vehicals, I've never had any of them start leaking oil after changing.
If you don't want to use syn. you should use Rotella for the additive package it has.
 
I've been told switching from conventional to synthetic could cause leaks but I can't recall being told that switching from synthetic to conventional will leak.
 
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I have a vibration that starts at about 30 in 3rd gear, but goes away in 4th. Could this be a u-joint going bad?

Yes it could be a U joint. It could be a lot of things. Crawl under your jeep and pull on your drive shafts. See if you have any free play on the U Joints. Also look for damage. If the drive shaft hit a rock it may not be strait anymore.
The vibration could also be tires out of balance. Maybe a worn ball joint. Jack up the front end an try lifting on the front tires to see if the ball joints have too much free play.
 
I changed the oil in my new to me 85' CJ7 for the first time the other day and the guy at the parts store told me that since the last owner used synthetic that I should to, or it could cause leaks to go back to conventional. Is this true or did I get took for a ride.

Also, I took it out today to go play and on the way home I noticed that I have a vibration that starts at about 30 in 3rd gear, but goes away in 4th. Could this be a u-joint going bad?
Have not learned these things yet and some help would be great.
Thanks.

Unless someone can produce actual evidence, I think your parts guy was telling you a story. Everything that I have read from the automobile manufacturers indicate there are no significant problems switching back and forth between conventional and synthetic oils. The only thing may be that most of their studies have been with relatively modern engines, rather than old Jeeps!

I also agree that your vibration sounds like a u-joint issue or a similar problem somewhere along the drive shaft.
 
I checked the u-joints, they are solid and looked at the drive line and I can't find any visible damage. I checked my ball joints and they are good, but found that the tie rod for the driver side is loose. Could that be it?
 
I checked the u-joints, they are solid and looked at the drive line and I can't find any visible damage. I checked my ball joints and they are good, but found that the tie rod for the driver side is loose. Could that be it?
Yes, a bad tie rod can cause the wheel to vibrate.
 
I checked the u-joints, they are solid and looked at the drive line and I can't find any visible damage. I checked my ball joints and they are good, but found that the tie rod for the driver side is loose. Could that be it?

BusaDave9 is right, a tie rod could cause vibration. I would also make sure the tires are balanced.
Back when Mobil 1 first came out, I remember someone telling me that if I started using synthetic motor oil, I couldn't switch back to conventional. Actually that's a load of :dung:. I've switched back and forth on many vehicles. I had a couple of Astro vans for work that I can remember switching at one time or another. I didn't have any issues.
 
The leak issue comes from using :dung: conventional oil, then going to a good quality syn. with better detergents cleans out the crud plugging those small leaks, and the leak starts. going back to conventional from syn. is no issue.
But use rotella or mobile1 if you do.
 
The leak issue comes from using :dung: conventional oil, then going to a good quality syn. with better detergents cleans out the crud plugging those small leaks, and the leak starts. going back to conventional from syn. is no issue.
But use rotella or mobile1 if you do.

So true.
Another issue is that the synthetic oil penetrates better. Usually that is an excellent trait for an oil but a synthetic oil will penetrate through an oil seal if it is not good as new. If the seals are in good shape then you can switch to synthetic oil without a leak.
 
I think that I am going to have to change my rear main seal now, because now I have a leak, but I'm hoping that is the only leak. But it is making it look like my t-case is leaking to. How hard of a fix will this be with no air tools? And how hard is it to change the seals in the t-case?
 
The Dana 300 is similar to the Dana 20 . I think thats what BrockGrimes meant. The T-150 is a 3 speed transmission that is often mated to a Dana 20 TCase.
As for replacing the main seal on CJs there is a brace below the oil pan that goes between the frame rails. This brace needs to be removed. Then remove the oil pan, remove the rear main bearing cap. all other bearing caps will have to be loosened. When you remove the rear main bearing cap you will see half the seal there. This will go half way around the crankshaft. The other half of the seal is above the crankshaft. You will have to press it out. This is why you had to loosen the other bearing caps: to release tension and allow you to drive the top half of the seal out. Once it starts to come out you can grab it with pliers and pull it the rest of the way out.

Here is a good link for reference:
Jeepin.com - Replacing your rear main seal.
 
I had my valve cover gasket leaking. You know those plastic things they put on 258s, hard to seal. Anyway, because of the oil leaking out, it appeared to be a rear main seal leaking. I replaced the valve cover gasket, no more leak. The oil at the bottom of the engine that appeared to be a rear main, went away also. So, before you go to the hassle of pulling everything that needs to be pulled to replace that rear main, make sure the oil isn't coming from the top of the engine.
 
before you go to the hassle of pulling everything that needs to be pulled to replace that rear main, make sure the oil isn't coming from the top of the engine.
Good point.
Those are both problem areas for leaks on the strait 6.
When I had a AMC 258 i6 / 4.2l I repeatedly repaired the valve cover until I replaced the factory plastic cover with an aluminum after market cover.
 

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