The 325 normally call for the 15W40 as you mentioned. The reason it is red is because of a dye they use so you can see it easier. I forget the part number of the dye, it comes in about 1 ounce bottles. We have it in stock in our shop.
The larger units call for ISO 46, however we use ISO 68 and also add the dye to it. Without the dye it is real hard to tell where it is on the dipstick.