Adding a little 80w90 gear lube to your 20-weight motor oil isn't going to make the viscosity go nitro on you. 90-weight gear lube is about the same viscosity as 30-weight motor oil. Gear lube also has a lot of antiwear additive, so it won't hurt things in that respect. The only negative I can think of is gear lube's complete lack of acid neutralizing ability (TBN) to deal with products of combustion. But since you only added ~1 teaspoon to your oil, it won't have a significant effect.