Not at all, the viscosity of the oil only comes into play in the final 30% or so of travel. I fine tuned my suspension using the oils in the front and rear. Front was a mix of 5 and 10 (7 was too light, 10 was too heavy so I "made" a 8.5
) and the rear was heavier that light, but lighter than medium. Just make sure if you are mixing your own that you stay with the same brand to avoid incompatibilities.
