I'm ready to change oil in the '26. I have 3 quarts of 30w and 1 quart of 10 w 30. When all 4 are in the sump does the oil become 3.3 w 30? I think I'm getting bored in this 95 degree heat this early in the season.
SAE 30w has the same flow characteristics at hot test as 10W30 in test environment used by the industry.
10W-30 flows the same as SAE10 when tested in Winter industry standard environment.
Mixing types of oil does not average them out. You just get a little of the cold low viscous chemical composition with a lot of 30W. Who knows what you end up with, one may actually interfere with the other. Let's hear from the chemical engineers on the forum.
Not sure about the affects of mixing, but oil is measured in viscosity, not weight. The W in 10W30 means "winter". It's a normal 30 viscosity in summer, but acts as a 10 viscosity in winter. I don't usually run my touring in the winter, so I go with a straight SAE 20 or 30 viscosity. My guess would be that it has no affect in summer since 10W30 acts as a straight SAE 30 in summer. Running it in the winter may be a different story though.
Joe is correct. The straight 30W is going to be thick as mud on cool mornings. It will be very hard to crank in cold weather.
All that really matters is that you change the oil often. That quart of 10w-30 mixed in with the straight 30 won't mean a thing. Every time you dump the oil, there's almost a quart of the old stuff left in there in the dippers and clinging to the bands, etc. 400 miles is a good number to shoot for.
Believe it or not, my Model T chose 5W30-weight. _When I use that stuff, the car doesn't creep forward upon starting the engine.