HowNot2 recently tested whether sunscreen weakens climbing rope:

Contrary to UIAA warnings that it might weaken it by up to 40%, it did not appear to. But will DEET? There are plenty of reports and anecdotal evidence of DEET doing bad things to plastics. But how bad, and does that include climbing rope?

40% is about the point at which the weakened sample would be weaker than a full-strength sample tied into a typical climbing knot.

>40% and 0%-40% both require that the result be statistically significant (2 sigma level); distinction between them based on raw result compared to rope MBS.