Synthetic oil and rear main seal leaks—the myth and the reality!

[Irun]

We've all heard the stories about synthetic oils causing leaks in our 2 part seals. Some say they do, some say they don't. Recently, I installed a new Rear Main Seal (RMS) in the TJ. When I was finished with the install, rather than buy more oil, I re-filled the Jeep with some high mileage 10W-30 synthetic leftover from a previous vehicle. Immediately after using synthetic, the Jeep developed a substantial oil leak from the newly replaced RMS. The leak was so bad that in a 120 mile round trip, oil ran down the length of the skid and was splashing droplets onto the rear axle housing. At rest, there was always a drop of oil hanging off the bottom of the bell housing.

Always wanting to test the synthetic vs conventional oil leak myth/legend, and not wanting to replace the RMS again, I went down to the local parts store and picked up two 4 quart ($13.69 ea) containers of Valvoline Premium Blue Heavy Duty 15W-40 conventional oil. After draining out the old oil, replacing the filter, and re-filling it with the conventional 15W-40, I took it on another 100+ mile trip. When I returned from the drive, I crawled underneath the Jeep and guess what? There was no oil hanging off the bell housing and nothing running down the length of the skid. I know I've seen this posted about a thousand times, but never experienced it first hand. Yes, this is only one sample, but synthetic oil causing RMS leaks is a real thing. At least in my case it is!

 

[freedom_in_4low]

Around Y2K or so, I put Mobil1 into my 1989 Ford Taurus SHO one time. Figured it was a high RPM engine so it ought to have synthetic. The seeping quickly turned into a major leak. I went back to conventional, and it went back to a seep.

That's been enough for me. I've put full synth in cars where factory specs couldn't be met otherwise (like ACEA A3/B4 5W40 recommended for my Mini Cooper S) but everything else gets conventional. EDIT: and usually the high mileage blend, because I don't believe in taking loans out for cars and rarely buy anything that isn't at least 10 years old

 

[pagrey]

I've never run synthetic and have had a RMS leak for at least 75k now. I used to switch between 10w-30 and 15w-40 conventional summer/winter and noticed a big reduction in the leak going to 15w-40. Now that my mileage is 150k I just use 15w-40 all the time and the leak is less than it's been in years. I think not only conventional oil but using diesel oil helps as well.

 

[Jerry Bransford]

I've never run synthetic and have had a RMS leak for at least 75k now. I used to switch between 10w-30 and 15w-40 conventional summer/winter and noticed a big reduction in the leak going to 15w-40. Now that my mileage is 150k I just use 15w-40 all the time and the leak is less than it's been in years. I think not only conventional oil but using diesel oil helps as well.

High Mileage versions?

 

[pagrey]

High Mileage versions?

Just regular cheap 10w-30, Valvoline or whatever was on sale, same with the diesel oil. Kirkland is my latest batch of 15w-40 (I think I used 10w-30 Chevron from Costco when they had it). Never high mileage although I hear that works.

 

[Jerry Bransford]

Just regular cheap 10w-30, Valvoline or whatever was on sale, same with the diesel oil. Kirkland is my latest batch of 15w-40 (I think I used 10w-30 Chevron from Costco when they had it). Never high mileage although I hear that works.

Switch to the High Mileage version of a conventional, it can't hurt and it'll likely help. It cured a massive RMS leak in my BMW.

 

[JMT]

I might try 15w-40 next time. I still can’t get the leak to stop with high mileage conventional 10w-30, though it is better.

 

[HornedToad]

This isn’t leak related but I have tried full synthetic in both my Jeep’s and each time the already present lifter tick got a lot louder.
As far as leaks go I recently did a very heavy deep clean of the engine on my tj. I sprayed all the thick oily crust with degreaser, let it soak, and then pressure washed it with a detergent mixed in. Now it leaks like crazy (I’m talking a 3 inch puddle from a single drive). I’ve got a new rms, oil pan gasket, and valve cover gasket that I need to get installed before I get in trouble with my apartment complex.

 

[Jerry Bransford]

This isn’t leak related but I have tried full synthetic in both my Jeep’s and each time the already present lifter tick got a lot louder.
As far as leaks go I recently did a very heavy deep clean of the engine on my tj. I sprayed all the thick oily crust with degreaser, let it soak, and then pressure washed it with a detergent mixed in. Now it leaks like crazy (I’m talking a 3 inch puddle from a single drive). I’ve got a new rms, oil pan gasket, and valve cover gasket that I need to get installed before I get in trouble with my apartment complex.

Switch to a High Mileage conventional engine oil.

 

[GASnBRASS]

Had a similar situation happen with an old Jetta with well over 100K. Decided one day to run full synthetic, same weight. In short order every gasket and seal was puking oil. Learned my lesson and have been running regular dino oil ever since. Synthetic might be great for bearings and rings but if you can't keep the oil in the motor then what's the point.

 

[IAmTodd]

Currently running Castrol Edge 5W-40 and no more leak. It leaked a little post RMS replacement with Penzoil 5W-30. I think the weight affects it more. I snagged Rotella T6 0W-40 for dirt cheap for the next change - I'm curious to see what happens.

 

[Irun]

I might try 15w-40 next time. I still can’t get the leak to stop with high mileage conventional 10w-30, though it is better.

Generally, I switch to a 15W-40 on anything over 150k miles. Although I usually use Rotella, this time around, because it was on sale at NAPA, the Valvoline went in.

 

[JMT]

Generally, I switch to a 15W-40 on anything over 150k miles. Although I usually use Rotella, this time around, because it was on sale at NAPA, the Valvoline went in.

I only have 95,000 miles on mine. Would it hurt anything to try 15w-40?

 

[Cornbread]

I only have 95,000 miles on mine. Would it hurt anything to try 15w-40?

I've ran Rotella 15w-40 in mine for the last 6 years and mine runs like a champ. The RMS will drip a couple drops until engine cools down then it stops. Hasn't gotten any worse in those 6 years either. Right or wrong I have ran 15w-40 in my lawn mower, atv's, Jeep, and pickup and never had any issues with any of them. For the farm, we buy it by the barrel from our local small town oil/fuel company and he recommends running it in everything I have.

 

[pagrey]

I only have 95,000 miles on mine. Would it hurt anything to try 15w-40?

Nope, I'd only think twice if it gets really cold. I think Jeep recommends it for some export model 4.0 engines where 10w-30 is difficult to find.

 

[nondem]

The first thing I did after bringing my 2003 TJ w/4.0 home was to change the oil and put "regular" Mobil 1 full synthetic 10W-30 in it...not a drop from anywhere in 500 miles so far. I did see in the carfax that the RMS was replaced about 5 years ago. It has 132k on it.

 

[pagrey]

The first thing I did after bringing my 2003 TJ w/4.0 home was to change the oil and put "regular" Mobil 1 full synthetic 10W-30 in it...not a drop from anywhere in 500 miles so far. I did see in the carfax that the RMS was replaced about 5 years ago. It has 132k on it.

Plenty of people run synthetic trouble free in these things, many on this forum. Obviously this is really aimed at people with leaks.

 

[Viking Jeeper]

I put high mileage Mobil 1 in my first TJ with 50,000 miles. Leaked right away. On my second and current Jeep the PO had just used Mobil 1 for the first time and it started leaking also. I use Valvoline blend high mileage now, no leaks.

 

[Wtrask]

I’ve used synthetic oil in mine since 2005 with about 40,000 miles and it’s never leaked a drop to this day.

 

[Fonz54]

I might try 15w-40 next time. I still can’t get the leak to stop with high mileage conventional 10w-30, though it is better.

Same for 10W30 Dino has reduced the RMS leak - I am going to try 15W40 too.