The venerable Allison M20. Bullet proof, allegedly. But maintenance is always important. Check the oil…. Book says hot (180-200º), whichis ironic as we are to keep the engines 180 or below, neutral and idle. (Fluid reads waaay high when cold). So the first time I did this the port engine was way high anyways and had tiny bubbles in the oil. (Aeration) This can be caused by a high oil level. So I took a quart out using the high tech “hose and outboard primer bulb” technique. VERY slow. Bring your phone and a pillow and make yourself comfortable. SB on the other hand read low. So I added a quart there. Check again next time they are hot. The objective is to get them right, and then some people make a “cold” mark on the dipstick. I like that idea.