Bought mine at the dealer, for my 1980 300D.

and then.... Checked the little valve (3 port plastic thing) on my valve cover.
It leaked, and I replaced it (about $40 from the dealer),
and my trans shifts better than ever.
Although I will still put in the K1 kit, I no longer have 2 - 3 flare.

Just my experience, your resutls may vary.

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250