Heath- Yeah, they have it on their website, my instruction manual says it and I've spoken with their engineering people about it. They say it will decrease the bias ratio a little but they are most worried that some syn oils have produced squealing noises. I gather these were some time back, so who knows with today's syns. Also, I think they are just carrying on with the same-old. My personal opinion is that since this unit was developed in the '70s, it was designed around mineral oils and they know the exact performance with those types of oils. They have had a few issues with syn oils over the years and AFAIK, they have not retested with syns to verify compatibility. I doubt they are inclined to undertake a bunch of testing just to change the oil recommendation. There are several types of "synthetic" oils, some that are quite different from each other (i.e. PAO vs Ester) so they could potentially open a can-o-worms and spend hundreds of thousands of dollars to test all the different oil formulas. If I were the boss, I wouldn't bother. I know of many people using or have used synthetics in Truetracs with no issues. Only know of one that had trouble... and he had the squeal (more like a squawk). But that was many years ago and on Mobil 1 gear oil.