Chrysler MDS & oil viscousity.

[dgallup]

I bought a used 2007 Dodge 1500 pickup with the 5.7 l Hemi and MDS (cylinder deactivation). It didn't have an owners manual or maintenance log so even though the oil looked pretty fresh the first thing I did was change it. I put in 5w30 Mobile 1. Since then I've down loaded an owners manual and done some research and it seems Chrysler only recommends the use of 5w20 oil with the MDS system. I've also noticed a low speed vibration that I don't think was there before (but I only drove it long enough to buy the oil at Walmart). The vibration goes away if I switch the transmission to tow/haul mode which I believe also deactivates MDS. I feel/hear the vibration at 1000-1500 RPM in top gear. I tried to demonstrate it to a co-worker at lunch but could not get it to happen. Of course, the ambient temp was about 25 degrees warmer so the oil would be a bit thinner.

So, am I doing the engine any harm with the 5w30 oil? Are the valves opening part way & causing a roughness or miss fire? I could just use the tow/haul mode until everything is good & hot I suppose. I'm planning on towing a 3000 lb boat many hundreds of miles the next two weekends. I would hate to throw away 7 quarts of clean Mobile 1 but that is cheaper than an engine rebuild.

 

[TheBlacksmith]

The thinner oils are normally to help achieve gas mileage ratings. The cold rating (5) is often beneficial in how quickly oil flows on a start up. I can't imagine it being a problem, but I'm not an expert on the Chrysler MDS system.

 

[dgallup]

I think the problem with the higher viscosity (spelled it right this time) is that the MDS prevents valve lift by bleeding off the pressure in the hydraulic lifters. If the oil is too viscose, the lifter may not fully collapse. Could this cause high stress in the valve train or rough running from the valves partially opening? I was able to duplicate the vibration again this morning. Switching the transmission in and out of tow/haul mode reliably made it stop and start vibrating.