[wms]

I too use Castrol Clearedge 6519 with very good luck. Sump life lasts between 4 and 6 months.

I've tried about everything and always come back to this.

It's not cheap but nothing good is. By the way I buy 6 drums at a whack, puts a real dent in the beer money. (about $800.00 a drum.)

None of the semi-synthetic mixes last forever, especially if you cut disimular metals.

If you want something to last forever try Castol Syntilo 9954.
I have one sump with 9954 in it and it has been in there for over 4 years, and still going strong.
All I've ever done to it is recharge it with a diluted mix to top off the tank.
The only pitfall to Syntilo 9954 is that it "pulls" all the plasticizers out of any acrylic parts (ie: sight glasses or windows) you might have and they will spider crack.

Two things I've found that will extend the life of any coolant are:
1) Never add straight water to the tank. (always use a diluted mix of water and coolant to top off the tank)

2) A $5.00 fish tank air pump in the tank will stop 90% of the bacteria growth and this is the smell we all hate on Monday morning (that is you guys who get weekends off).

There's my 2 cents (and worth ever penny, NOT)