Understanding Oil Weight Numbers

Do you want to spend the money on oil, or gas?

If it's going to leak anyway, it comes down to putting thicker oil in the crankcase and getting worse gas mileage but having a smaller puddle, or using thinner oil, getting better mileage but having a larger leak and having to buy more oil.

As a last resort, swell the gaskets with some additive.

Lyn,

I suppose that makes sense. I never considered that the thicker oil would decrease gas milage. Is it a significant amount?

I am using a seal repair additive as I stated in my question and it has helped.

weldeng449,

I think it matters over time. Maybe bobc or some of the other mechanics has an opinion.

I'll take a wild guess that it might be 5%-10% better with the thinner oil. Don't know if that's enough to worry about, or not.

How handy are you.

http://jeepin.com/features/rearmain

Oz,

Actually, I'm very mechanically inclined. In fact, I've already attempted to change the rear main seal myself. Unfortunately I didn't have a proper lift to place it on and had to jack it up in the driveway. Therefore my access was limited.

The seal is a 2 piece design. I could not get the upper portion off the crankshaft without causing damage. So, I replaced only the lower portion and hoped for the best. The improvement was dramatic, yet not complete. It still leaks a little oil. Unfortunately when it does, it is right atop the exhaust cross over. Therefore, even minor dripping causes the Jeep to resemble a mosquito pestacide spray trunck in June! Smoke billows from underneath. If it we a matter of just keeping the oil topped off, this would be a non-issue with me for this aging Jeep.

But thanks anyway for your suggestion and weblink.

David

Unfortunately when it does, it is right atop the exhaust cross over. Therefore, even minor dripping causes the Jeep to resemble a mosquito pestacide spray trunck in June! Smoke billows from underneath.

You've probably thought of it, but I'd look for a way to insert some sort of (probably metallic) combination diverter / heat shield to keep oil off the exhaust cross over--let it drip directly to the ground. (Maybe two or more layers so the oil doesn't smoke on the diverter (keep the diverter cool).)

RHK,

Funny you mention that. Actually I HAVE created a sheet metal "tent" if you will that I plan to hose clamp onto the cross over. I figured that with a 1" air gap to insulate the heat, the drip will not get hot enough to outgas. I've not installed it yet but am hopeful. Maybe I'll make another post to let everyone know how it works!

I wonder what the guy at the auto parts house will say when the new owner of my Jeep (once sold) will say when the new owner brings my "tent" in for a replacement? Love to be a fly on that wall!

Thanks for the suggestion!

David

You're welcome! Hope it works. If still too hot, you may need a 2nd "tent" with an air gap between the two.

You could put your name on the tents, and sell replacements to the next owner ;-)

Was the rear seal rubber, or rope like?

Rubber. Two piece. Tried like heck to get the top piece out but could not do it while laying on my back under the Jeep.

At the time I did take it in to get an estimate to have the seal changed at the dealer, they said they wouldn't do it because my crankshaft had play in it and it would just leak again. How would they know that if they had not removed the oil pan? I tried to wiggle the crankshaft by trying to jiggle the harmonic balancer. I couldn't detect any noticeable movement. Neither end play nor radially.

End play could be aggravated by a manual clutch. An automatic should not wear out the thrust bearing any faster than the main bearings on the engine. (they are one piece.) The technique I favor for the upper half is to use a small screwdriver about 1/4 " wide to force the seal closer to the crankshaft. Do both sides of the upper after a few minuets of this it should be able to slide around the groove that it is in and rotate to the bottom.

Is there a strong suction from the PCV valve hose? When the hose is removed from the valve, the engine should race quite a bit. If not try cleaning that vacuum hose out. Good luck.

If its a rubber seal they make a seal puller

Let you get the point into it and leverage it out a little at a time. Until you can get a good hold on it with pliers.

Thats excellent. Thanks!

That tool works great for removing a metal clad rubber seal from a bore. But the seal used on the Jeep is 2 half circles fitted into a groove machined into the engine block. And don't forget that the crankshaft is still installed in the engine. This is what the rear seal looks like. Smaller piece in the block.

Seem to be experiencing some pasting problem. I'll try another way.

That seal remover is great for metal clad rubber oil seals that need to be removed from a bore. A rear main seal on the op's Jeep is a split afair that fits into a groove cut into the block and main cap. And remember that the crankshaft is still in the engine.

Yes Bob. You got it! Couldn't get the top piece out. If I try it again, I'll use the tool you suggested.

Thanks!

I dont know about oil weight but....the numbers stand for flow-resistance.

In 10w-40 the 10w is the flow rate of the oil tested at 0 degrees C; the 40 means that this particuler oil will at 99 degrees C, have the viscocity [flow rate] of an straight SAE40 oil tested at 99 degrees Celsius. So the viscocity of a straight oil drops very much as the temperature of the oil increase. Additives in a multigrade oil like 10w40 causes the oil to thin out less with increase of temperature.

Best is to choose an oil with the higher end-number ...say 10w-50. If the engine is old and the oil presure is down then it will be an option to switch to 15w-50 oil. Remember the viscosity of any brand of oil numbered as an SAE 10 can have a viscocity of between SAE 6 and SAE 14 to be labelled an 10 oil!

You did not state your location, or which engine you have. If I were you, I would first get u under the jeep with a do it yourself car wash cleaning wand. Clean off as much of the oily mess as you can. Drive it home, and crawl under it and look for the actual wet spot. Many times repair facilities will sell the worst case job to cover their butts. Many times the oil pan is leaking, and can be tightened. If the pan is leaking even though the bolts are tight, you can seal non moving parts with silicone rubber. Just clean both sides with brake cleaner and a rag. Then bridge the gap with the silicone. If the rear main is the cause, squirt a little brake fluid at the rear seal. Look at the oil pressure sending unit also. The senders are prone to leaking, especially the ones used for warning lights. Another thing to check is the PCV system. It uses manifold vacuum to remove crankcase fumes and burn them. If this system is not removing sufficient volume of crankcase pressure, the crankcase pressure will find some place to escape from. And the crankshaft seals are the lowest point. If you need help testing this let me know.

If all else fails, 20W50 oil is out there, but you may be better off with straight 40 weight oil. Good luck.

When I lived in Singapore I had on old Mercedes 2 litre. The mechanic told me to put straight 40 W in and I had minimal leakage and no problems. The temperature never dropped below 25 Celsius (77 Fahrenheit) so cold starting was not a problem. You will not get away with this when the outside temp is close to 32 F unless you have a very good battery.

5w, 10w and so on. The w stands for winter.

At 99 degrees C a straight 40 has a viscosity lower than that of 20W-50 [Note this is a 20 oil at 0 deg. C and this oil at 99 degrees will have the same viscosity as straight 50 oil at 99degrees]

A straight oil will cause more engine wear during warmup to normal temp. And cause more engine wear at hight temp. because it loses to much viscocity.

The only time I would use a straight oil is say a 20, is when I want an engine to wear in quickly after an engine rebore.

jurie

Agreed. My comment about straight 40 possibly being better was because,there is the possibility of the OP being located in a cold climate, where the 20w-50 would be less viscous at the low temperatures.

Sorry to contradict Bob,

At cold temp. say 0 celsius The viscosity of 20W-40 is 20. Straight 40 is 40 and less oil will spray on cylinder walls, causing more wear.

I do not know of an aplication where a straight oil would be better than the right grade multigrade oil.

jurie

It is not the oil spraying on the cylinders that is the concern, it is the rate that the oil is seeping out of the rear main seal. And for that reason I believe that the straight 40 oil would be less likely to seep out of the rear main IF the ambient temperatures were colder, like 40f or lower.

If you decide to take the plunge and replace the seal on your own, it would be a great time to replace the oil pump, if it has not been done before.

Please tell me why we are talking of oil WEIGHT no. if we discuss viscosity

Mercury is heavy= Weight/mass, but the viscosity is very low = pours easily.

Because the OP has a rear main seal that leaks. He wanted to know what different oil viscosity ratings would have on the leakage rate. Commonly an oil that is not a multi viscosity product will be referred to as " straight 30 weight oil". In effect, interchanging the terms "weight" and "viscosity".