I don't know which oil is best but I have two recommendations:

1) oil testing
2) include transaxle oil

I have been using an oil testing service to check the transmission oil. Unlike engine oil, transmission oil is scheduled for change only every 60,000 miles and has no oil filter. I changed mine at 53,000 and had a real eye-opening experience with the silicon levels way too high. Scroll down a bit from that web page and you'll see what happened to the viscosity of that worn out oil.

It costs about $15 for a basic oil test. My recommendation is to also send off a virgin sample when you change so you can compare it to the used oil later. For $30, you will know what came out (and any potential problems) and have a baseline for your next test.

Now I'm am interested in the oil performance across different temperature ranges so I've added an additional $5 viscosity test at 100 (C). With two points and the Andrade formula, I've been able to compare relative oil viscosities.

Oil vendors usually have their oil specifications posted somewhere including the 40 (C) and 100 (C) viscosity measurements. Using the Esko URL, you can make your own plots and draw reasonable conclusions about the relative oil characteristics.

I am not a lubrication engineer but the arguments for synthetic oils make a lot of sense. A synthetic oil will have a more uniform molecular weight which should lead to uniform performance between batches. The problem is we don't know the fundimental lubrication property, 'film-strength'.

The one oil characteristic I am most interested in, film-strength, has not been readily available from the affordable oil testing services. The film-strength is a measure of how well the oil keeps parts apart, the fundimental lubrication property. Viscosity has more to do with what it takes to pump and push the oil about. But when viscosity changes a lot, the oil has been substantially changed and one can expect that the film strength has suffered. It is film-strength that keeps metal parts from rubbing and wearing against each other.

I don't have an oil vendor recommendation other than make it a synthetic. I would recommend backing it up with oil tests to confirm it hasn't changed significantly between oil changes and to look for unusual wear patterns. If you should find an affordable 'film-strength' testing service, let me know.

GOOD LUCK!

Bob Wilson

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

Automatic, stock, backup car.
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