The volume of lubricating oil stored in the sump of an internal combustion engine for a vehicle is in significant excess of the volume of oil circulating through the engine at any one time. The circulating oil, drawn from the sump, may be rapidly heated during its passage through the engine, but the excess volume remaining in the sump dilutes and cools the circulating oil as it returns to the sump. By separating the oil volume into a portion which is circulated through the engine and a second portion which has only limited opportunity to mix with and cool the circulating oil the circulating oil may attain its operating temperature more rapidly. Once the stored volume of oil in the engine has also reached its operating temperature the circulating oil and stored oil may be recombined.