Tamiya 87114 retarder anyone?

I appreciate that, but it is "guaranteed" to work & I have ofter wondered if adding X/Y brand retarder to Tamiya acrylic might cause an issue with the retarder that is already mixed with their thinner?

I haven't tried the Tamiya retarder, but there's any number of artist's acrylic retarders (eg. Golden, Liquitex, Jo Sonja's)  which have been used successfully for many years. I imagine that the Tamiya product is the same thing with a brand-name price tag.