I'm trying to help a friend save some money by changing the oil on her 1998 BMW Z3. She said the dealer has always put synthetic oil in it. When I look through the manual it only refers to special oils individually approved by BMW (low friction lubricants) are most preferable. I would rather put dino oil in it to save money if that will be acceptable. What type oil and weight would you recommend for her car. The car is used in Southern California and experiences weather typical of that area with an occasionl trip up to the mountains which can drop below freezing and sometimes out to the desert which can go above 100 degrees.