What is considered an acceptable increase in viscosity for gas powered vehicles (cars). My feeling is it needs to stay in grade to be normal and if its more than +/- 25% of original viscosity then its critical and an oil change is needed. Yet others say it can go up one grade and still be OK, but needs to be changed if it goes up two grades.

I would agree to +/- 20% change in viscosity. By viscosity I mean ISO VG viscosity grade. Pls do not mis-interpret SAE grade in the relationship.

The ISO VG is arranged in such a way that at the lower values it can go across "grades" but at the higher values it is likely to stay within the same ISO VG grade.

Hussam Adeni

Donald,
May be it will useful for you:
http://www.natrib.com/appnotes/app30.htm
Good Lack,

Mikhail,

Excellent reference. Many thanks.

Hussam Adeni