oil

[tahoe healey]

I haven't asked a dumb question for a while so I'll ask now. I like using Castral oil and since it is cold here I use 10-40 in winter and 20-50 in summer. What happens if I mix them? 10-40 and 20-50 make no difference in starting. Will it cause a difference in the carb damper wells?

 

[kch]

Yeeeeep, I think so. Since the 20-50 is thicker, there will be more resistance for the damper in the carbs, and that can make a difference in throttle response.

 

[Dave Russell]

Hi TH,
I don't think mixing the engine oils will hurt anything as long as they are the same brand.

Attached is a chart for oil viscosities. Use the right hand temp. scales. Viscosities are given for 100 degrees F & 210 degrees. In this case you would use the data for 10 wt. oil & 40 wt. 10 when the engine is cold & 210 when it is hot. Or compare to 20 wt. It also compares various gear oils which are rated at 100F.

Since once the engine is warmed up the hot radiator air is warming the carbs, they will likely stay at a fairly constant temperature. A cold engine requires more carb damping than a warm engine so a single viscosity oil does just the right thing by thinning as it warms up.

SAE 20 is normally used in the dampers year around. I've also had good luck with ATF. Folks don't usually use multi vis oils in the dampers or seasonally change it. Interesting concept though.
D