Would I ruin Winsor and Newton Series 7 Watercolor brushes if I use them with oil paint???

Agree with Mia. My watercolor teachers both told me that if you use watercolor brushes to paint oils in, the solvents that have to be used to clean the brushes will render them unusable for watercolor again. The solvents take the outer cuticle of the hair off, or close the hair cuticle completely, so that it cannot expand again. The opening of the hair cuticle in watercolor brushes is what gives them their water carrying capacity. You will ruin that with the solvents.

I have to wonder why you would want to use watercolor brushes for oils? If you bought the brushes yourself, you know the price of them. If you didn't buy them yourself, you need to know that the FINEST Kolinsky sable brushes run in the hundreds of dollars for just one large brush.

I just opened the sale catalog from one of the art houses I buy supplies from online: Cheap Joe's Art Stuff. The catalog lists Series 7 W&N Limited Edition pure Kolinsky sable Size 12 brushes ON SALE for $141.79US. When paying full retail price, it is listed as $465.00US. Why would you want to take the chance of ruining a brush set that expensive?

As a rule:

Painting ruins your brushes. It is just a fact of life. Cleaning them thoroughly help prolong their lifespan but in the end they will wear out.

As a rule:

The more a brush costs the shorter it's lifespan (but the better they paint as well)

As a rule:

Once You picked a medium (acrylics, oil, watercolor) there is no way back for your brush.

To answer your question:

Yes, eventually you would.

Forget Winsor Newton it is too pricey for a beginner and Cotman although "student quality" sucks too. Go online to Cheap Joe's or Dick Blick and pick between American Journey (Joe's house brand) reasonable compared to WN or Van Gogh which is very inexpensive for beginners and has decent pigment strength. It is the ratio of pigment (color) to medium that has a lot to do with price. The less pigment the less expensive the paint,l but the lousier result. All watercolor tends to dry 3 shades lighter but with Cotman or student grades the colors are not only lighter they lack luminosity.

I still have some Kolinsky sables given to me in the 1960s,by my old art teacher when I went to art college.They have been used for all types of paint.I do not use them for mixing,only applying the paints and inks and wash them carefully in white spirit after use,cleaning them in warm water and washing up liquid before rinsing them and leaving them upright to air dry.For scumbling techniques I use a more resilient hog or nylon brush,these sables are for fine lining and wash and glaze techniques only.

If you aren't going to do too much oil painting, then I would buy some cheap paint brushes to use rather than risk damaging your really nice brushes.

If you use them you will never be able to use them on watercolor again

You should never mix brushes.