1959 Mercury 55-A

[Hurrel]

I bought a 1959 wood boat yesterday. It is a Cadillac hull and has a 1959 Mercury 55-A, 40 H.P.

The owners manual says to mix at 24:1 (D-50 formula)
but someone hand wrote in the manual to use 50:1.

I looked at a post on this website that referred to a Mercury service bulleting that says it is o.k. to use 50:1 in the older Mercury motors.

However, the engineer side of me is extremely curious as to "why" only 1/2 the oil specified in the manual is needed and also to confirm the 50:1 mixture to be absolutely sure.

I really hate to ruin a good engine.

Thanks

 

[JB]

Re: 1959 Mercury 55-A

Use 50:1 with TC-W3 certified outboard oil. The reason for the change is the improved quality of outboard oil since 1959.