I have an LBZ and I want to change to a true synthetic engine oil. My owners manual recommends SAE 15W-40 as prefered above 0 deg to 100 deg, and 5W-40 from 100 deg to below -18 deg . I have not been able to find a 15W-40 synthetic, but have no trouble finding 5W-40 which is not recommended for temps over 100 deg. What is the downside of using 5W-40 year around, our summer temps go to 110 deg at times and rarely below zero in the winter?

Check this site http://www.mobil.com/USA-English/Lubes/PDS/GLXXENCVLMOMobil_Delvac_1_ESP_5W-40.asp. Delvac 1300 Super 15W-40 (dino) and Delvac ESP 5W-40 (syn) are both diesel oils that are CJ rated.

There are *no* downsides to using 5W-40 full synthetic year round. Only *up* side..... The operational temperature range of Mobil Delvac 1 ESP 5W-40 (and other full synthetic CJ-4 diesel engine oils) is vastly superior to mineral oil based oils. You are MORE than covered using a 5W-40 full synthetic..
George Morrison, STLE CLS

x2

I have an LBZ and I want to change to a true synthetic engine oil. My owners manual recommends SAE 15W-40 as prefered above 0 deg to 100 deg, and 5W-40 from 100 deg to below -18 deg . I have not been able to find a 15W-40 synthetic, but have no trouble finding 5W-40 which is not recommended for temps over 100 deg. What is the downside of using 5W-40 year around, our summer temps go to 110 deg at times and rarely below zero in the winter?
For godsakes man,dont do it!:eek:
Your truck will last longer get better mileage and run better....;)

I had the same dilemma a few years ago with my diesel motorhome. I called Cat. tech. service and they told me that the only reason for not recommending 5W-40 synthetic was that they had not tested synthetics in Cat. engines. The reason for this was the lack of demand. He said over-the-road truckers and heavy-equipment operators did not use synthetics because of the quantity and price. However, he said using it would not void their warr. and as far as he was concerned synthetics were far superior to dino oils. I used Rotella T 5W-40 full synthetic in my 330 Cat. for 6 years and never had a bad test result. The 5W-40 synthetic has every bit of protection as 15W-40 and has in fact been upgraded by Amsoil to their recommended oil for 2007 and newer diesels. Of course, any new oils can be safely retro'd to older models.:D

Here's what Mobil says about Delvac ESP 5W-40 (I don't work for them or sell the stuff)):

"Mobil Delvac 1 ESP 5W-40 is recommended for use in all super high performance diesel applications, including modern low emission engine designs with Exhaust Gas Recirculation (EGR), Aftertreatment systems with Diesel Particulate Filters (DPF’s) and Diesel Oxidation Catalysts (DOC’s). These applications include the following:

On-highway engines operating in both high speed/high load and stop-and-go conditions

Off-highway engines operating in severe low speed/heavy load conditions
Virtually all diesel powered equipment from American and European equipment builders

High performance gasoline engines and mixed fleets
Refrigeration units"

i use rotella synthetic. never would use anything different. Check at alocal chevy dealer to see what wights they have. our supply changes depending on type of year as to what weight what we have

Thanks everyone for the input. I bought a case of Mobil Delvac 1 ESP 5W-40, I'll put that in tomorrow. I am the second owner (2007 LBZ) it had 2300 miles on it. There was a run in the clear coat after repairing some fender scratches and a cigarette burn in the drivers seat. The dealer agreed to take care of these when my FREE 1st oil change was due. Since this is my first diesel I read the manuals cover-to-cover and request all the recommended 5K service (engine oil, spin on trans filter, and tire rotation). They had my truck 2 days and did give me a truck to drive. The dealer charged me $57.00 for the transmission filter/labor and an additional charge for the rotation, the oil change was FREE? When I picked up my truck and left their lot I was on a very busy 4 lane during rush hour, as I applied the brake at first light a plastic cover from above the emergency brake fell behind the brake pedal. I did manage to just stop before I hit someone. They had shorted out something when the repair to the seat was done and failed to secure the relay cover properly. I have no confidence that service work at the dealer will be done by competent techicians, so I have geared up to start doing this again. The Allison service center charged me $8.33 for that same filter, so I have already started to save. I don't mind paying a reasonable price for service, the tire rotation was reasonable, the trans filter was not.
I have ranted enough, I do like my truck, its the dealer service I do not like.
Thanks again for all the oil spec info, this is really a good site.