My question is that if I am using 10-20 wt mulitiviscosity oil and I put in an Oil Additive how does it not interfere with the mulitiviscoity of the Oil?

I say that because usually Oil additives are thicker. Raising the Oil viscosity would also decrease leaks and maybe help seal worn rings/cylinders to hold in more combustion.

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84 300D, 82 Volvo 244Gl Diesel