What oil viscosity is needed for a Manitou Minute Expert fork? Some people say it's 5w some say it’s 15wt and what weight is best to use?
Hard to answer from manufacturer docs I have been able to find. A couple of things point me towards 5Wt. The manufacturer sells 5Wt and 3Wt oils on their web site, nothing near a 15Wt. Most internet posts, including one from singletracks.com (a fairly reputable site), suggest 5Wt.
There are two required - the semi-bath oil is 5W40 and the fork oil 5Wt.
It is likely the correct amount of oil in the fork is far more important than oil weight.
Not sure where you saw a 15Wt recommendation - it might be some people have found 15Wt gives better fork Proformance for them.
Overall, I do not believe it matters much. Wrong oil will have the shock performing poorly or unable to be adjusted as damping will be affected, but changing the oil to a different weight will fix it and there is low risk (I am tempted to say no risk) of lasting damage.