Power steering help!

[w211estater]

Hi everyone. Power steering fluid on the w211 e500 4matic estate needs replenishing. I've located the reservoir, with a little help from this forum. MB says to use their proprietary fluid, but I was wondering if this is in fact necessary. I saw some threads about this but it seems that most of the time the answers were hidden in private messages! Which power steering fluid is recommended? Many thanks.