In this paper we study perturbed Ornstein-Uhlenbeck operators \begin{align*} \left[ \mathcal{L}_{\infty} v\right](x) = A\triangle v(x) + \left\langle Sx,\nabla v(x)\right\rangle-B v(x),\,x\in\mathbb{R}^d,\,d\geqslant 2, \end{align*} for simultaneously diagonalizable matrices $A,B\in\mathbb{C}^{N,N}$. The unbounded drift term is defined by a skew-symmetric matrix $S\in\mathbb{R}^{d,d}$. Differential operators of this form appear when investigating rotating waves in time-dependent reaction diffusion systems. As shown in a companion paper, one key assumption to prove resolvent estimates of $\mathcal{L}_{\infty}$ in $L^p(\mathbb{R}^d,\mathbb{C}^N)$, $10$. We prove that the $L^p$-dissipativity condition is equivalent to a new $L^p$-antieigenvalue condition \begin{align*} A\text{ invertible} \quad \text{and} \quad μ_1(A) > \frac{|p-2|}{p}, \,1

16 pages, 2 figures