Viscotech 494 The SSI index is the degradation and loss of polymer structure after processing or operation. Viscotech 494 The lower the SSI, Viscotech 6540 the more synthetic the polymer becomes and the less it degrades. Viscotech 494 Currently, the most preferred polymer in engine oils is OCP polymer, which has a low SSI value and is easily melted, such as Viscotech 6540 with an SSI of 20-22. In synthetic DPF engine oils, the most preferred is Viscotech 494-L Stern Polymer with an SSI value of 0-5. PPD (polymethyl acrylate) Viscotech 6540