I had a bit of a look at a couple of the oil thickening additives a few weeks ago and I found the viscosity figures of one of them. It worked out that if that was mixed with 5L of 10w-40, it would make the oil into at least a 10w-60 ie way to thick for most engines. When I say at least a 10w-60, the viscosity of the additive was described as 'in excess of', so realistically the oil could end up as a 20w-70, not a grade you see often, or ever, as it's just too thick.