Cold start studies were conducted on 10 different engines (including duplicates of one type), and six different multigrade oils ranging from SAE 0W-30 to 25 W-30. As expected, the results showed that for all the engines, the minimum starting temperature decreased with decreasing winter viscosity grade. The 4.0L I-6 engine had the highest minimum start temperatures (MSTs) for all the oils, while the lowest MSTs were distributed over seven of the other eight engines tested. The difference in the starting characteristics of the 4.0L I-6 engine were attributed to the fact that this engine design was relatively old (early 1980s) compared to the rest of the engines in this program (1990s). Another interesting observation was that, excluding the 4.0L I-6 engine, there was no effect on the MSTs due to engine displacement, number of cylinders or configuration. This is contrary to older studies which showed that as number of cylinders increased, minimum starting temperatures decreased. Improved starting characteristics of these modern engines are attributed to improvements in engine technology.
An excellent correlation was observed between minimum start temperature and cold cranking simulator (CCS) viscosity for the modern engines. In addition, the CCS viscosity for these engines at the minimum start temperatures was 2 to 4 times higher than the present SAE J300 limits. In contrast, the MSTs for the older engine design (4.0L I-6) appear to follow the present CCS temperature and viscosity limits. The oil viscosity at the MST for the 4.0L I-6 engine was relatively constant (3,000 to 5,000 mPa-s) throughout the whole temperature range, while with the newer engines, limiting viscosity decreased with decreasing temperature (25,000 mPa-s at -19°C to 7,000 mPa-s at -37°C). This requirement for lower oil viscosities at lower temperatures is attributed to a need to compensate for increased internal engine friction with decreasing temperature.
Comparison of these results with those from the 1970s shows that the older engine design (4.0L I-6) follows a similar pattern (comparable CCS viscosities and no temperature effect) to the old 4- and 6-cylinder engines, while the newer engines parallel the results for older V-8s.