This is what I would do: If the previous owner(s) ran synthetic, I would just continue. If they did not, I would see if you get and leakage and if it uses any oil with a standard high-grade motor oil first. If no leaks and doesn't use much, then I would try to switch to synthetic and see how that works. If you get a lot of leaking and have to add a lot, the synthetic won't help you much at this level of mileage. If the engine is well maintained and tight, the synthetic could help you get even more miles out of it. On the other hand, high grade standard oils now days will protect your engine very well as long as you change frequently as recommended by the manufacturer.

 

So many oil myths.........I'd like to see that list of engines (any engine) that "were not designed" to use synthetic oil or designed only for the use of conventional oil.

 

Man that's for sure. Just go to arfcom and read the arguements between the folks who say using Mobil 1 on an AR15 works great and those who say only "gun" oils should be used on guns.

 

Some misinformation & myths in this thread. OP I would suggest you do your oil research on a site where real experts discuss the issue. Sometimes people are so in love with their own experiences that they share it as gospel, forgetting that thousands or even millions of others have different results.

My experience with the XJ so far: I bought it in 2001 with 79k original miles on the clock. Switched to Amsoil shortly after (it now has about 145k or so), following some research ... alright, the truth is, I believed the hype on the Amsoil site. I don't recall the mileage at which I switched to synthetic, but I did notice some new "seepage". This is due simply because it cleand out the sludge that was preventing the dino oil from leaking. Basically, some oil keeps the area slightly damp, but I have never had to add oil, and there is absolutely nothing on the ground. After 9+ years nothing has changed & the 4.0 runs as strong as ever, with zero issues. My seals have not deteriorated & the rear is still slightly damp (rust inhibitor!). It spit back at me once when I was forced to put dino oil in during an oil change when I didn't have enough syn available. Coughed when I started it - immediately shut it off, drained, and went to get some syn. That's about it. I'll continue to run syn until it drops dead - or I do.

My .02 & YMMV.

 

Conventional and synthetic oils are both terrible for flat tappet cam'ed motors. Zinc was removed from car type oils ~3-4 years ago and Diesel type oil year or two ago. There is not common over the counter oil that is suitable for use in a 4.0. Around a year so ago our autozones and advance auto parts still had "costal fleet" diesel oil, it was the last I could find of the good stuff (that is not as friendly for the envirnoment), it was the last of the stuff at the time, I imagie it is gone now. I bought 5 of the large cases of the Rotella T from Sam's Club when the last of it was available about a year and half ago. To get by today you have to buy "racing" oil, look for something that has "for offroad use" on it, there is a type of Valvoline classic oil that can be special ordered, or you have to use a convention or synthetic oil with a zinc/zddp additive. Some chain stores carry ZDDP Plus. GM EOS can be purchased from GM brand dealerships. The "cheapest" way is to buy Crane cam break in lube. Its hard to find, you generally have to order it from Summit Racing and my last order stayed on backorder for ~4 months. A bottle is pricy but it it has a huge amount of zinc, but instead of adding a entire bottle of EOS per oil change you basically can add a capfull of Crane super lube to each change.

 

I was burning 1 qt of oil every 200 miles when I first bought my XJ last October.. the second oil change I did, I used Mobile 1 High Mileage 10w30.. and noone will ever convence me to use something else. That stuff worked wonder. After the first use of Mobile, oil burnoff went to 600 miles, then with the second it was virtually less than a qt every 1000 miles. 3rd oil change had almost no burnoff...

 

What supports this??????????????

 

bobistheoilguy.com is great site, here is another one that I think might be of use.

 

This time with the link.

http://www.carbibles.com/engineoil_bible.html

 

about the zinc issue, i know certain oils advertise they have zinc in them, i cant recall if it was amsoil's diesel oil or if they have "regular" oil+zinc. I personally run amsoil in my jeep and havent had any problems, i get it at wholesale so i just use it. I know there is a 3rd party oil analysis company that amsoil recommends using, you take a small oil sample from your dipstick tube and send it in. You then get a report card printout from them with parts per million of various metals and impurities (like antifreeze), and a letter grade. It has quiet a bit of info on it and is pretty detailed.

Get the report back and theres no signs of excess metal and all grades are good, they say keep running the oil. I know the main distributor for lower michigan was running a dual bypass oil filter setup and had over 100k miles on his diesel. He then had the engine taken apart completely and had an ad in amsoil's magazine and all the components looks amazing for how many miles were on them. All he had to do was change the oil filters at the recommended intervals. Now im not saying we can do that with out old engines, but im thinking of sending in an oil sample and seeing what kinda grade my engine gets, will be interesting.

If anyone wants me to scan one of the report cards, ill see if i can find one of them and upload it.

 

It is incredibly irresponsible for auto makers to recommend an oil that is bad for their new cars/trucks ... don't you think? You should send them this paragraph and hope they change their wicked ways.

 

yup, if you dont send your oil off and get it tested you dont know what you oil is doing or any other valuable info that comes with it

 

FANSTASTIC and very thorough scientific analysis and explanation

and yes....i quoted myself

 

......and the myths just keep on keepin' on. This article debunks (there's that word again) the ZDDP/flat tappet cam myth several times. Who's had oil related engine failures from using SM rated oil?

http://www.zddplus.com/TechBrief2%20...Oil%20Myth.pdf

 

They don't. Roller cams are used now and don't need zinc. Oils at the time they were produced were fine to run the engines. I own several classic cars, and it is a big deal.