In a munual transmission changing the fluid is not that critical, but I would still do it. I suspect that the mechanic may have put too heavy a fluid in. This is all the more puzzling, since synthetic fluids are normally thinner during cold weather for the same amount of protection.
To my knowlege there are no lines to plug in a manual trnasmission, unless it has an external cooler, which is rare.
I would check the invoice an see what really went in there. It sounds suspicious.