Forget mixing oil with underseal, it will not mix properly or stick, if you have access to a compressor then purchase a schultz gun and one tube of underbody schultz. The gun is around a tenner from Machine Mart, purchase ordinary underseal, the paint on type and thin this to the consistency of that in the tube with white spirit. Refill your old schultz tube and underseal your vehicle with your set up.
Clean and keep the old schultz tube for future use, tubes of schultz are about £7 each, a gallon of ordinary underseal about £10, you will use 4 tunes of schultz to complete your vehicle.

Once this had dried, about a month, then paint old engine oil over the problem areas, and spray into the chassis internal sections.