I have the NuSyntec in a few cars and was hoping NOT to use my reserve stocks for topping-off purposes. The NuSyntec is ester-based, so I figure "what would clash the least with it" as a topper. The Pennzoil HM is already ester-blended, so shouldn't it and the dino oil in it be more compatible with MY synth anyway? More compatible then say the odd case I have of Exxon SuperFlow Synth, an orderless PAO. Usually I have been adding Chevron Supreme to whatever oil I am running. I maintain 4+ cars so I am not spending $5 everytime one is a little low on oil. Penzoil HM, yes? FTR- my friend used it in his 100k Mazda today and commented via cell call "it seems great".