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It's a bit complicated, but I'll try to hit on the highlights. Non-detergent oils were recommended for older tractors for two reasons. (Keep in mind that we are speaking of recommendations from 50+ years ago). First of all, there obviously were no detergent oils available 60 years ago. Secondly, when they came out with the first detergent oils, they had a high ash content. As the oils burned away, they left an ash deposit. This deposit was as bad as the sludge problem. Tractor manufacturers just decided that non-detergent was the lesser of two evils. The detergent oils of today don't have the problems associated with the first detergent oils. The oils of today don't burn off nearly as rapidly, and the detergent formula is different. In a modern engine, or a newly overhauled older engine, detergent oil is the only way to go. Now, to change from a non-detergent to a detergent oil on an older engine - it's a bit more complex. If you have a sludge buildup from non-detergent, the detergent oil might free this and cause clogging. This can usually be eliminated by changing the oil and filter more often after switching to detergent oil. I would say more often is defined as maybe three times as often as before. At least for the first 200 hours. Dropping the oil pan and removing the sludge will help greatly too. There are also stories of seal leaking after switching to detergent. This is caused by a bad seal in the first place. The sludge is what kept it from leaking. If this happened to me, I'd replace the seal. Most people have switched to detergent with no problems. In the case of the D17, it might already be using detergent oil. If you already are using detergent, don't change back. I oversimplified things here, but hope this helps.
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