I have a John Deere LX176 lawn tractor and am confused about its hydrostatic transmission oil. The manual states that the hydrostatic transmission oil should be "SAE 10w30 engine oil with API classifications of SE, CC or CD" but I don't know what those classifications mean. Would just regular oil do or could I even use some multi-purpose automatic transmission fluid?
SE was from way back (circa 1979), so any off the shelf oil should be well ahead of that at a minimum. The current standard is SN. Since I would assume you will not be changing this oil very often, I would highly suggest you purchase some thing like Royal Purple or some other high grade synthetic engine oil. I know it's a little expensive, but considering you won't be changing it very often, the expense is well worth it.
DO NOT use automatic transmission fluid (ATF) - that fluid is NOT compatible with the oil that's in there now, and it may even wreck your seals.
If John Deere wants you to use regular ol' motor oil (which is part and parcel of their specification), that's what you should use. Depending upon your climate, you may need 10W30 or you may be able to get by just fine with anything from straight 10 to straight 30.
If that tractor were mine... I'd use the exact same oil in the transmission as in the crankcase, nothing special.