JimsGX

Hi All,

Has anyone experienced oil leaks after switching to synthetic motor oil from dino oil? I switched to Mobil 1 on my 91LS 1k miles ago. Now, at 87,500 miles, I have a rear main seal leak. It started two days ago.

I did some searching on Google and found quite a few reports of rear main seal leaks when using/switching to synthetic motor oil.

I suctioned out a quart of the synthetic through the dipstick and added a quart of "Bar's Rear Main Seal Repair" yesterday. Drove the car about 75 miles and when I checked this morning, the leak appears to be gone.

When I found the leak the other day, I called my indy mechanic and made an appointment to have the rear main seal replaced. Now I'm not so sure if I should bother, if sticking with dino oil would keep the seal from leaking.

trukn1

At 87,500 miles, your car is due for it's 90K service anyway in which the seals should be replaced. Not to mention, if they are still original, your car is 18 years old and these seals DO NOT last forever. Am really impressed that your 91 has such low mileage. Anyway, there is no such thing as a "miracle in a can". That so called rear main seal repair may "work" for the moment, however I can almost guarantee that when the leak does come back, it will be worse than before. As far as switching to fully synthetic oil, with the relatively low mileage on your car I don't think it would hurt any, however you need to get the seals changed due to the age. On a "higher mileage" car, I would not suggest it. There are many pro and cons to going synthetic. You just need to figure out which arguement works best for you. I, personally, run my 92 with 176K on the clock, and I run Mobil 1 semi-snythetic and have not had a problem with leakage.

JimsGX

I'm surprised to hear this. I didn't know that the main seals were on the maintenance schedule to be replaced at 90k miles.

Och

iTrader: (3)

No, which seals are usually involved in the leakage? I replaced my crank seal about two years ago, and I'm about to replace cam seals and valve cover gaskets. Is the main rear seal the only potential problem after I'm done with these?

trukn1

At one time or another they should be replaced. Normally people get them done with the 90K service since a great deal of work is being done then. 90K service, people get their timing belts, water pump, tranny service, and a host of other work done. Makes it a little easier to chane/get to the seals since a good deal of the engine is opened up anyway. The problem with the rear main seal is that you pretty much have to drop the tranny to change it. At 18 years old your seals, if never been changed, are just worn out from age.

Och

iTrader: (3)

Right, and Im wondering if I switch to synthetic (my car has about 130k) after Im done with cam seals and valve cover gaskets, are there anymore potential leaks besides the main rear seal?

trukn1

I, personally, run a semi-synthetic and have had no problems since I got my car at 154K. I know, on mine, a host of seals were changed. I may very well still have the rear main one left to do, since you need to drop the tranny to get to it; however I have not experienced any oil leakage. If you change you gaskets and seals, and your car is performing well when you do the switch, you should be fine. Just try to avoid going back to dino oils from a synthetic.

JimsGX

I just came across this article that sounds somewhat believable to me that explains why switching to synthetic oil can cause leaks to appear. Let me know what you think...

http://bestsyntheticoil.com/info/seals.shtml

rob22

I bought my 94 LS last summer, it had a leaking rear main. Oil loss was small, maybe a couple drops when parked overnight. I switched to Valvoline Longlife and the leak has changed to more of a "weep" with no oil drops when parked overnight. So far, I've run Longlife 5W30 and 10W30, both seemed to keep the leak pretty well contained. Car currently has 235K miles.

Still running it with the leak but plan to repair later this summer. The car needs a new EGR tube, engine mounts, and transmission mount; all of which I'll do with the rear main. I'll also replace the seal(s) at the front end of the transmission. Consider doing these items with yours when you get the rear main replaced, as you're already paying for the labor to drop the tranny.

Hope this helps-

avalon42

iTrader: (1)

You can check the rear mail seal to see if it is leaking. There is a small cover plate in front of the transmission bell. You take it off, and then you can insert a rag, paper towel, or anything to see if there is engine oil in the bell housing. If there is, you have the rear main seal leak. If not, then it's oil flying down from somewhere else in the engine.

My friend and I were checking, and it turns out my vehicle had a full service job south of the transmission bell.

skperformance

iTrader: (6)

switching to syn after years and years of base dino stock will 99% start to leak. It is because the engine is dirty inside and has old debris covering seals which will now be cleaned off . The seal is now dry and brittle from heat and no lube so it leaks. People blame the oil but it is just dirt , the less frequent oil changes the worse it is.

PureDrifter

iTrader: (10)

i've switched 4 cars to synthetic, most had over 80k miles, one had over 120k miles, and NONE developed any leaks

mind you, all of them were toyotas

JimsGX

Maybe your just lucky or I'm just unlucky!

From what I've been reading, I agree with SKPerformance. I think normal deposits/contaminants in the dino oil probably develop around the seals and help to fill in any gaps, but at the same time, the dino oil blocks fresh oil from fully saturating the seals so they start to dry out / shrink. The full synthetic cleans those deposits away and exposes the shrunken seals.

From what I've read, many years ago they didn't have the science of synthetic mixtures correct and it was common to have leaks when switching to synthetic. I also read that adding 3-4 ounces of DOT3/DOT4 brake fluid to the oil (I won't be trying this) will make them swell back up to normal size if they haven't deteriorated to a point of no return.

Its also possible that given a little time after the switch to synthetic, the seals will restore themselves and stop leaking if they aren't too far gone.

Having Lexus specify 7500 miles between oil changes (on the first gen anyways) probably doesn't help matters either. I know my old LS was well maintained by the original owner, but they did wait the recommended 7500 miles between oil changes. Given this fact and the fact that they were only putting 5k miles a year on the car (87,500 miles now at 18 years old), the oil was only getting changed once every year and a half!