MG Car Repair: 1966 MGB vented gas cap, gas cap, gas fumes

Question
A mechanic that worked on my MGB 1966 said that my gas cap was not
seating properly leading to gas fumes.  It seems after he put another MGB gas
cap on it, it did not run well.  Does this mean that the gas cap actually can
make a difference in its performance?  Also, what oil should I be using in this.  
The previous owner said a straight 30 oil is all that he used in the 25 years he
had it.

Answer
Hi Janet,
If he put on a non vented cap it can effect the way the engine runs by making it difficult for the fuel pump to supply fuel to the carburetors. The 66 B did not have a vented tank so it needed a vented cap. Easy test is to remove the cap (when the fuel level is low) and drive the car, if it runs ok then, the cap is a problem.

There is nothing wrong with straight 30w oil, however most straight 30w oil was non detergent oil and that means the inside of the engine walls are coated with a build up of residue. If you now put in a detergent oil, it will wash much of that down into the oil which can get by the filter and through the bearings and causing damage.
You can pull the valve cover and look at the inside of the valve cover and the valve train to see if there is any build up. If not you can use any oil you like. I had a 66 B and I always used 10w40 Castrol. If you decide to switch oil, you should only run the new oil about 500 mi and change it and the filter. Then you should be able to run 2000 mi between changes but always change the filter every time you change oil. Use engine oil in the transmission and 75 to 85 hypoid oil in the differential. Use engine oil in the carburetor top pots.
Howard