If it is something that you put into your Engine Oil and you have Hydraulic Lifters it could have unstuck them or is keeping the pumped up at idle.

As an example in the past before multi-viscosity Oils were used and there was that ticking Hydraulic Lifter noise owners would add something like STP. One of the things STP did was to make the oil thicker which helped keep the oil pressure inside of the Lifters themselves as well as the thicker Oil helped take up some of the clearance in the bores on the Engine which the Lifters were riding in.

I do not know what type of cleaning effect STP has. But, the other type of addative was mostly a solovent designed to remove deposits inside of the Lifters and of course other Engine parts in hopes of freeing them up.

I have always wondered if putting addatives in Multi-viscosity Oil interfers with how it functions so I have never put addatives in it.

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84 300D, 82 Volvo 244Gl Diesel