According to my maintenance manual Volvo recommends 15w40 in my 1996 960. Does that mean I should use diesel engine oil or a 15w40 oil for gasoline engines?

 

Do you have a diesel engine? I use 10w30 full synthetic for my '96 960. I live in the south so it doesn't get to cold here. Do not use diesel oil.

 

Thats pretty incorrect. Diesel oils actually have just as good if not better lubrication properties as gasoline oils, and significantly better additive packs. Most notably is the additional ZDDP (zinc) which is necessary for valvetrain protection, and usually a bit more Moly which helps with ring & cylinder wall wear. Based on the chart in the manual from my '960, I'd say a 15W-40 is a bit thick for winter months due to the cold temperature viscosity of a 15W oil. I run Rotella 5W-40 syn in mine. One of the best synthetic oils on the market, and only $19 a gallon at Walmart.

OTOH, if you're in a warm climate year-round and don't do alot of short-trip driving, a 15W-40 might be ok. For winter months though, I'd recommend a 5w, maybe 10W oil at the thickest.