You will likely have trouble finding synthetic 10w40. Synthetic 40 wt is usually 5w40, which would be fine. The 40 is the operating temp. viscosity index. The 5 or 10 is the cold index and should not matter. Keep in mind that all 40 wt oils fall into a range of viscosities, the 40 wt range, rather than meeting a single viscosity number. I would likely go with a name brand 10w40 dino oil as an experiment. If it solves your problem you can go back to the synthetic again.

There are also oil thickeners like Lucas, STP, Motor Honey, etc. that could be added to the oil you are currently running, however you would not know what your oils viscosity really is. As an experiment it could tell you if the gauge flicker is due to too thin an oil.

The thin oil is used by auto manufacturers to get the best fuel mileage figures on the window sticker. I understand that the govt. says that they have to recommend the oil they use in the fuel economy tests. I personally would not use 20 wt oil in any engine. 30 wt oils are better for longevity in any new engine.

When I was a kid there were no multivis. oils. We ran 30 wt in the summer and switched to 20 wt in the winter, and an engine would only go about 100k miles, due in my opinion to the use of the 20 wt. Engines have improved slightly since then and oils have improved dramatically, but thin oil still causes more wear than thicker oil.

 

Nelbur you do a great job explaining things and also giving the history on older vehicles using thinner viscosity makes sense. I wish I knew this 15 years ago when I drove off the lot. Hoping it helps. I just changed the oil so I may try the thickener first as an experiment.

Richard