I recently changed my oil vicosity from 20/50 to 10/40 on my 89' veezy, with 214k. Surprisingly, I noticed an increase in mileage- but also an annoying, random, not loud groaning sound from the engine at idle(sounds like increased idle, without being so). It goes away whenever I apply the gas, but returns when I sit at idle. Should I go back to 20/50? The engine still doesn't leak a drop and runs very cool, with good oil pressure. Ideas?