Standard Fluid or motor oil lubricates.

Additives do not lubricate. They help oil to break down more slowly.

The less oil you have, the less lubrication.

The more additives, the longer the blend lasts.

My car has had Mobile1 Full Synthetic since I purchased it.

I use standard ATF and Diff oil.

To my understanding, if you have used regular motor oil for a long period of time (>50K mi) it is actually a bad idea to use synthetics or fluids that have alot of detergents in them. Here's why: there is a "sludge" that builds up in your engine over the years. This stuff is yucky and full of nasty sediment. When you drain your oil and add normal oil the sediment is not disturbed. When you add detergents or synthetic motor oil this "sludge" starts to be circulated through your engine. This can cause more problems than good.

I thought that the SVX called for synthetic oil anyway. Am I wrong?

I also read a TSB about transmission fluid and differential fluid additives. It said that Subaru does not reccomend you use anything except standard OEM type fluids. The systems were not designed for any other viscosity. Allthough, adding these will not void the warranty. (who has a factory warranty today anyways?)

While we are on the subject of fluids in our beautiful Subaru's, I would like to advise AGAINST having your transmission flushed and filled. I had this performed about 5 weeks ago and my Subie rests on a lift as you are reading this. She'll still move but I'm not going to drive her untill she's a 4 spd again. (I still have 1st & 2nd gears) The transmission wont engage 3rd or 4th gears. I'll git er done though!

Good luck out there!

-Elliot