I think most riders will notice their suspension characteristics change due to severe ambient temperature variations.

I notice that even my TTR250 with stock suspension suffers from this problem. I use 20wt fork oil because in the very high daytime temperatures found in my work (highest so far is 49degC shade temp) the standard 10wt is worthless. I also find that the shock's compression and rebound action is less during very hot days compared with cooler mornings of, say, 15degC.

As mentioned above, any oil or grease I'm familiar with thins as it gets hotter. So I don't think it's necessarily the fault of your Ohlins. Just how it is. All suspension settings are a compromise of some kind imo.