There are significant differences in cooking properties between milk and cream. The more fat, the less likely a dish is to curdle, so you can be taking a substantial risk if you substitute skim milk for heavy cream in a cooked recipe. But, in general, a recipe that calls for milk or whole milk is not using it for its cooking properties – rather to add flavor and richness to the dish. The use of skim or reduced-fat milk in place of whole milk will produce just the result you expect – a thinness in both taste and texture. Depending on the amount of milk called for, the difference may or may not be very meaningful to you.
You can certainly boost the amount of fat in the milk you have on hand to approximate whole milk, which has about 3.5% fat. Half and half has about 10% fat, light cream has about 20%, and whipping or heavy cream runs between 30% and 40%. We recently got into trouble by bragging about our math skills, so by first admitting that these numbers may be wildly inaccurate, we believe you would approximate whole milk by mixing one part half and half with three parts of 1% milk, one part light cream to seven parts 1% milk, and one part heavy cream to 15 parts of 1% milk (or 1 tablespoon cream to 1 cup less 1 tablespoon of 1% milk). As a practical matter, the proportions don’t matter all that much – add a little cream and you boost flavor and richness.