I tried to buy some new engine oil today but, for the first time, I having a hard time finding synthetic 15w-40 diesel engine oil for my '99 Dodge Ram 2500. The truck has 264,000 miles and I've owned it since it was new. I usually run Amsoil 15w-40 Heavy Duty Diesel & Marine Oil or the Mobil 1 equivalent. All the stores now seem to only carry the 5W-40 synthetic oil which I thought was only for the newer diesels. Can I safely use the 5w-40?Thanks. ----- Jim

5W is a lower viscosity usually for colder climates but a multigrade (i.e. 5W-40 or others) is suitable to operate in a broad temperature range.Just be sure the oil you choose is rated for diesel engine serviceI am guessing that the oil manufacturers may just be making their product suitable to cover the bases for both older and newer diesel engine requirements and therefore we are likely to being seeing 5W-40 replace the 15W-40 stock.

5w-40 is fine, it will most likely be a CJ oil instead of a CI like the Amsoil. CJ doesn't last as long, but that may not effect you if your not doing extended drain intervals. 5w-40 is an OEM recommended oil in winter, which is why it's available.

i ran 5-40 mobil for 2 years and now swiched to 15-40 ams oil with bipass kit supposed to be good to 25k and 60 k with sample, but i only put 5k a year on it my truck is 2002 2500 6 spd, with ams oil and bypass after 1k it's still clean and with 5-40 mobil after 500ml starts to get darker. i think 15-40 has more zddp in it for non emmision trucks.