When I first got my Toyota Hiace with a D4D turbo diesel, I was amazed to find that the oil dipstick didn't work. No matter how I tried to use it, the 'business end' always came out smeared with oil and completely unreadable.
Several online discussions mentioned this problem, with users strongly divided into two camps: those who had nothing but trouble and those who had none. There didn't seemto be any obviously-identifiable factor causing the problem, but someone mentioned that he had found twisting the bottom two inches of the dipstick strip slightly gave a partial cure.
I tried this on my vehicle, the degree of twist being a compromise that still allowed the strip to flex around the bends of the guide tube without getting jammed edgewise-on. To my surprise, it worked and the dipstick is now sufficiently smear-free to be readable.
The probable cause of the divide between working and non-working dipsticks then dawned on me: The guide tube on a non-turbo D4D is fairly straight; whereas, on the turbo engine fitted in the Hiace van, it has an 'S' bend to negotiate around the turbo unit. In flexing around the bends, the dipstick slides against the walls of the tube and the oil mark is smeared.
It would be interesting to test this theory by asking members of this group with D4D engines to say whether they have experienced dipstick problems and whether these were with turbo or non-turbo versions fitted to the Hiace van.