T-SQL WHERE IN clause: one parameter containing multiple INT values will return type conversion error [duplicate]

I have a SP which looks up a product (only one) by its productid just like this (it's a very simplified version of the query)

ALTER PROCEDURE [dbo].[MYSP]
@productid int
AS BEGIN
    SET NOCOUNT ON
    select * from products where productid = @productid
END

EXEC MYSP 10

In some special cases I need to provide two or more productid but I am NOT allowed to pass more than ONE parameter to the SP so I tried to change the parameter from INT to VARCHAR and do:

ALTER PROCEDURE [dbo].[MYSP]
@productid varchar(10)
AS BEGIN
    SET NOCOUNT ON
    select * from products where productid IN (@productid)
END

EXEC MYSP '10,20,30'

However it will return me a conversion error because the productid column in the table is INT.

There is a workaround for this?

Thanks!