Synthetic or not synthetic, that is the question

[Mjollnir]

The reason for this posting is supposed to be simple, but the issue has literally driven me nuts for months since I purchased my '99 Disco II with 76K back in Oct. '09 (it now has 107k). The issue I speak of is motor oil.
I first started using Castrol GTX High Mileage 10W-40, but then switched to Mobil Clean 5000 High Mileage 10W-40 up until 100k. I then used Pennzoil High Mileage 10W-40 once, and switched to Castrol GTX High Mileage 20W-50 for the last two oil changes. I kept switching brands to find the best anti-sludge and anti-wear combination, but only Castrol has a patented formula. I switched to 20W-50 for the spring and summer for extra protection against wear. As far as engine condition, I confirmed the gaskets were replaced, and there is no evidence of oil leakage anywhere on the block itself.
What I wanted to do for quite some time is switch to Mobil 1 High Mileage 10W-40 all year round (they have a 15W-50 but not for High Mileage vehicles). It has additional anti-sludge additives and anti-wear additives that I would require as I intend to keep the Disco for a very long time, and I am familiar with the benefits of Mobil 1. However, I was advised that using any synthetic oil would cause seal leakage, except for one enthusiast who has been using Mobil 1 for years and swears by it.
Also, the cost. There is all of a $1.03 difference between six quarts of Mobil 1 that I can purchase at discount and the six quarts of Castrol 20W-50 I have to purchase at my local auto supply store. For that little difference, I would like to switch to Mobil 1.
If anyone has experience with this issue, please let me know as I prefer to use a full synthetic.

[Cottingham]

Thor - I wrestled with that when I had my 99' Disco II. I drove that thing to 189,000 (with the normal valve job around 125,000) and never converted to synthetic. It won't make your head gaskets leak - they just do that on their own ever 60,000 miles or so.

I've had my 04' Disco for a while now and have close to 95,000 miles - still no synthetic. It's for sure an owner's choice, but I'm not sure there's any real advantage to running synthetic as long as you're getting the oil changed on a regular basis. It may make a few people feel as though they're taking better care of their green-oval friend, but the Disco engine is a TANK.

One thing I will side with is using Castrol whatever.

[Mjollnir]

Thanks for letting me know. I used to run Mobil 1 in my other vehicles, and I did notice smoother, more efficient engine performance. Also, as the DII is getting up there in miles, I figured a full synthetic would be easier to flow through the engine, which would result in less wear on the oil pump and faster protection of engine parts.
I use Mobil 1 75W-90 gear oil for the f/r diffs & transfer case, and what a vast improvement over whatever was in there before. I genuinely believe in its performance, but when someone who has 25 yrs. experience working on Land Rovers says don't use synthetic, you tend to listen to them, which is why I'm running 20W-50 Castrol in it now.
Months ago, the cost difference between the oils was great, but now it's peanuts. That's why I'm even thinking about it.
I have also researched Castrol Syntec and Castrol Edge, but neither one comes in a High Mileage formulation like Mobil 1, and Edge is not available in a 10W-40. So, I'll give Mobil 1 a shot next time round. I've got enough leaks to deal with, I would perfer not to have one in the engine bay.

[98 HOO]

Very intersting reading on oil if you haven't seen it before.

http://www.ferrarichat.com/forum/faq.php?faq=haas_articles

[dcwhybrew]

Here's another good thread regarding oil for your rover.

http://www.discoweb.org/forums/showthread.php?t=54420

Bottom line is pick an oil with a high temperature high shear rating of 3.5 Personally, I use Mobil 1 Synthetic 15w50. Its HTHS rating is 4.6 or something. I have also used Rotella synthetic 5w40

[Mjollnir]

dcwhybrew & 98 HOO:
Thank you both for the links to the other forums pertaining to synthetic oils. Both were very informative, but I have to say as far as the Ferrari thesis, it was quite wordy, yet interesting. Much more involved than I would get, but quite knowledgeable from an avid enthusiast who cares about his vehicle and hopes others can gain from his experience.
For the discoweb link, the issue is that not all synthetic motor oils are the same. For high mileage vehicles, you must use an oil with a seal swell additive that increases the thickness of the seals. Some high mileage conventional oils have it in a 10W-30 grade, but only one synthetic oil has three different grades of high mileage motor oil with the seal swell additive, and that is Mobil 1.
The reason seals leak is because they become thinner. A synthetic oil which does not have the seal swell additive will blow right through it. Even the Mobil 1 15W-50, as thick as it is, will not prevent leakage.
On the '99 DII I have, for the last 30,000 miles, used only conventional high mileage oils, both Castrol GTX and Mobil Clean 5000. Yesterday I switched to Mobil 1 10W-40 High Mileage and that is what the Disco will have exclusively for the rest of its life.
After all my research, as well as the advice of those not only on this forum but who are familiar with Land Rovers, I made this decision based on the benefits of Mobil 1 (less wear on the engine and less oil flow resistance) and the fact the seal swell additive is of a higher quality than that in Mobil Clean 5000 HM, and I never had an issue with oil leakage.
So, there it is. A synthetic can be used, but not one w/o the additive to prevent seal leakage. The poor individual who used Royal Purple, which does not have it, found out the hard way what not to use as it was not designed with the requirements of older Land Rovers in mind.
Thanks again for all the help!
-Thor