Vauxhall Vectra C 1.8 53 Reg - Power Steering Fault - wellman

I have owned the above vehicle from new and todate it has covered 56000 miles, recently the power steering has tended to be stiffer and feels lumpy at times than in the past particularly at low revs and also on full lock. I am told by a garage mechanic who has experience of Vectra C's that this is likely to be caused by low power steering fluid in the reservoir which will reqire topping up. Looking in the Haynes manual it says the fluid is Vauxhall Opel fluid 93 160 548 and I have been told that this is only available from dealers if they do the complete repair. Can anyone advise if general PS fluid available from Spares suppliers will mix with the original or does it have to be from a main dealer.

Many thanks



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