"He said that they had seen non-JD brand oil gum up the fuel shutoff - "even the synthetic oil"."
Is it possible, sure, anything is possible. I AM a lubricants salesman for Texas Refinery Corp. Was this "gum up" problem caused by any good quality oil, probably not. Did it happen with synthetic oil, I highly doubt it. Could it have happened with a low quality oil that had low quality base oil stocks, low viscosity index, low flash/fire point, sure, that's possible.
Like was said previously. Most salesman work on commision, I do. Problem is many salesman stretch the truth or do anything they can to get the sale. Any statement I make when I do a demonstration is backed up by fact in writing with labratory data to support it. I doubt your salesman with the "gum up" problem had anything in writing to prove what he was saying. Basically, if it sounds like hype it probably is.
Is JD oil a good quality oil, sure it is. I can't think of one OEM manufacturer of tractors, auto's or truck/tractors that uses junk oil. Are they the best there is, no. OEM lubricants tend to be a thin line between cost and quality. They can't be to expensive or the normal every day customer would flip out over the price. Conversely, they don't use the cheapest either or large numbers of equipment failures due to low quality lubricants would give the company a bad name. They have to find the happy medium.
