MCP2515 CANBus - pause in reading messages

I've successfully connected to my car's can bus using this shield. I am using coryjfowler's mcp_can library.

The 5-minutes cycle working with data is the following: MCU communicates with car's CAN BUS, collects data (takes around 2-3 minutes). Then it should do another time consuming job (1-2 minute). For example - send data over a network. While sending data it stops receiving data from MCP2515.
As far as I found, if the message is still in MCP2515 receive buffer and another arrives - it will be just overwritten (with a flag risen somewhere).
Are there any errors sent to a car's can bus if the message is not acknowledged by my MCU?
I am using NORMAL mode to be able to send and receive messages.
Should I switch MCP2515 to a loopback mode to stop MCP from receiving messages each time when MCU is busy with another task?