The use of the higher viscosity will make shifting harder and will wear out the main shaft in the transmission, as it will not flow into the needle bearings.
"I receive many complaints from people about how hard it is to shift their manual transmissions after an oil change. In every case, going back to the originally recommended 75W-80 or 75W-90 oil fixes the problem. There are even some dealers who put 75W-90 in and have complaints.
A local Suzuki dealer has a problem with a transmission that they could not "solve". They were about to order a new one under warranty, but the service manager called me for advice. I sent him some 75W-80 as a sample and solved the problem, becoming their supplier.
Not only is the 75W-90 too thick, but in many cases has the old traditional additives, which hurt the brass or fiber synchronizers. The 75W-80 and 75W-85, (and some 75W-90) do not have that high amount of sulfur/phosphorous extreme pressure additive, depending more on magnesium sulfonate and others."
Source- Richard Widman, Automobile Specialist
