[2.2 Straight six]

it thins the oil. in the same way white spirit thins paint.

oil needs to be of a certain thickness in order to do its job properly, this is based on engine stresses, engine speeds etc..

if the oil is too thin they it's not as good a lubricant as the engine need.

think of it like this, an engine needs really thick heavy oil to make sure all the parts are properly lubricated. now if you use a light thin oil it won't stay on those parts are easily, on rotating parts it might be "thrown" off and won't do it's job properly.

it depends.