Trans and gear oil?

[JimT]

I have 75/90 in my trans and rears. I didn't change it myself so the shop (TA) chose the oil (I think it's Mobil 1). No complaints though. I run Northeast/New England and mostly northern states so there's a good chunk of the year with at or below freezing start-up temps. Honestly, when you consider the average cost per mile for gear oil it doesn't make a huge difference to my bottom line.

Now, when it comes to engine oil I add the non-synthetic Super Tech 80/90. At one change every 1-2 months that cost would add up fast.

Rawze's thread on gear lube in engine oil.

As for your target engine oil change interval, you should be basing it on your fuel mileage instead of a random number.

I take the average fuel mileage for the current oil, subtract 2.0 and multiply by 3000. That gives me a target mileage for my next oil change.

Example: If I'm averaging 6.5mpg for the current oil, based on hard numbers tracking my fuel mileage, then 6.5-2.0= 4.5 * 3000 = 13,500mi interval. Add that number to the odometer reading when I did the last oil change and that's my target.

This calculation corresponds to the intervals Rawze suggests in his thread below and works really well as a formula in my fuel mileage tracking spreadsheet. (I created the formula based on Rawze's chart. When it differs slightly it does so conservatively. Better safe than sorry)

Rawze's thread on bypass oil systems and oil change intervals.