My Infiniti had an oil filled rear differential mount. It's was made that way to 'supposedly' make the car quieter. The problem is that it was one piece with the sub frame and as such was a $2000 repair when it leaked, or a $1,200 repair (with labor) to swap it out with a solid Whiteline bushing, after the original oil filled bushing was punched out. - I sure hope my 2020 CX-5 engine mount is is more traditional to replace